Banana cake with a cinnamon and hazelnut heart

At home there are always two or three bananas going black before anyone gets round to eating them. That's what this cake is for.

It's my take on the banana cake we've always made, with two things that change it completely. The first is brown butter, which gives it a caramel and hazelnut depth. The second is hidden: a layer of brown sugar, cinnamon and hazelnuts that forms a heart inside the cake and only appears when you cut it.

The hazelnuts aren't there by chance. In Tarragona, where Erreke is based, hazelnuts are the nut we've always had, and they get on wonderfully with banana and cinnamon.

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Three details that make all the difference

Almost black bananas. The riper they are, the sweeter and more fragrant. If the skins are still yellow, wait a couple of days or use the oven trick I share in the notes.

Brown butter. Cooking the butter until it turns golden toasts its milk solids. It's five extra minutes, and it's what makes people ask what's in it.

Eggs whisked with the sugar. Whisking them until pale gets air into the batter. With this much fruit, that's what stops the cake from being heavy.

The recipe: Banana cake with a cinnamon and hazelnut heart

• Prep: 25 minutes (plus 15 for the butter to cool)
• Baking: 55 to 65 minutes
• Rest before turning out: 10 minutes
• Serves: 12 to 16
• Tin: 2.5 litres (Clásico or Bloom)

Ingredients

For the cinnamon and hazelnut heart

• 50 g plain flour
• 65 g light brown sugar, firmly packed
• 5 g ground cinnamon (2 tsp)
• 35 g toasted hazelnuts, chopped
• 25 g unsalted butter, melted

For the cake

• 380 g plain flour
• 5 g bicarbonate of soda (1 tsp)
• 4 g fine salt (1 level tsp)
• 8 g ground cinnamon (1 tbsp)
• 1 pinch of nutmeg
• 270 g caster sugar
• 90 g light brown sugar
• 100 g unsalted butter (for browning)
• 55 ml sunflower oil
• 4 large eggs
• 10 ml vanilla extract (2 tsp)
• 360 to 380 g very ripe banana, peeled and mashed (3 large or 4 medium)
• 160 g plain unsweetened yoghurt, Greek or regular

For the tin

• Softened butter
• 1 tbsp plain flour

Method

Before you start

1. Brown the butter. Melt the 100 g of butter in a saucepan over a medium heat, stirring all the time. First it will foam; then it will turn golden, smell of hazelnuts and you'll see toasted specks on the bottom. At that point take it off the heat, pour it into a bowl and let it cool in the fridge for 10 to 15 minutes. It needs to cool without setting again.
2. Prepare the tin. Brush the whole inside with softened butter, not melted, getting into every ridge and around the central tube. Dust with the flour, turn the tin so it coats everything and tap out the excess upside down.
3. Preheat the oven. To 165 °C, top and bottom heat, no fan. Put the shelf in the lowest position and don't leave any trays inside, so the heat reaches the whole tin evenly.

The batter

4. The heart. Mix the flour, brown sugar, cinnamon and hazelnuts. Add the melted butter and stir with a fork until it looks like wet sand. Set aside.
5. The dry ingredients. In a large bowl, sift the flour with the bicarbonate of soda, salt, cinnamon and nutmeg.
6. The wet ingredients. In a stand mixer or with an electric whisk, whisk the eggs with both sugars for 3 to 5 minutes, until pale and fluffy. Add the cooled brown butter, the oil and the banana, and mix only just enough. Finish with the yoghurt and vanilla.
7. Bring the two together. Add the dry ingredients a little at a time, on the lowest speed or with a spatula, and stop as soon as you can't see any flour. If you overmix, the cake turns out tough.

Assembling and baking

8. In layers. Pour half the batter into the tin. Scatter the filling over it in an even ring, keeping it away from the outer wall: sugar stuck to the metal burns and grips. Cover with the rest of the batter. The tin will be about three-quarters full; don't fill it any higher.
9. Bake. At 165 °C for 55 to 65 minutes. From 50 minutes, push a skewer into the centre, avoiding the filling layer, which always comes out damp. It's ready when the skewer comes out clean.
10. Ten minutes, then turn it out. Leave the tin on a wire rack for exactly 10 minutes. Put the rack on top, flip it confidently and lift the tin straight up. Let it cool completely before slicing.

Notes and tips

Bananas not ripe enough? Put them whole, in their skins, in the oven at 160 °C for about 15 minutes, until the skins turn black. Let them cool, peel them and use them.

Brown butter without the drama. Use a light-coloured saucepan so you can see the colour clearly. It goes from golden to burnt in seconds, so take it off the heat as soon as it smells of hazelnuts.

No hazelnuts. If anyone at home has an allergy, leave them out of the filling. The cinnamon heart works just the same. You can also swap them for walnuts.

To serve. A light dusting of icing sugar and that's it. It's even better the next day, once the banana has spread its moisture through the whole crumb.

Storage

• It keeps for 4 days at room temperature, in an airtight container or well covered.
• If your kitchen is warmer than 25 °C, keep it in the fridge and take it out half an hour before eating.
• To freeze, in slices wrapped in cling film, for up to 3 months. Defrost at room temperature.

Which tin to use

This recipe is designed for a 2.5-litre tin. I make it in the Clásico, whose straight flutes really show off the golden banana crust, and in the Bloom. It turns out just as well in the rest of our 2.5-litre moulds: Flow, Blaze, Plait, Twist, Gleam, Flare and Vortex.

In a smaller tin, only if you adjust the quantities. Made as written, the recipe would overflow the 2-litre Cascada and Amour and the 1.8-litre Espiral. To use them, multiply every ingredient by 0.8 for the 2-litre moulds and by 0.7 for the 1.8-litre one. Beat the eggs and weigh them: a large egg is about 55 grams out of its shell. And start checking the bake about 10 minutes earlier.

The rule that never fails, whatever the tin, is not to overfill it. In our guide to tin capacities and conversions you'll find the volume of each one and how to convert any recipe.

What I used

• The Clásico mould or the Bloom mould, both 2.5 litres, in cast aluminium with a double non-stick coating.
• The set of baking trays with rack. The rack is where the cake spends its ten minutes before turning out, and where it cools afterwards, with air circulating underneath.
• An olive wood board or a teak board to bring it to the table and slice it in front of everyone.
• A pastry brush. If your tin has a few years behind it, have a look at how to look after the non-stick coating so it keeps releasing like the first day.

Frequently asked questions

Can I use frozen bananas? Yes. Defrost them in a sieve, discard the liquid they release and weigh them once drained. If you don't drain them, the batter will be too wet.

Can I make it without browning the butter? Yes, just melt it. The cake is still good, but it loses the caramel and hazelnut note that makes it different.

Why does the cinnamon filling sink to the bottom? It usually happens when the first layer of batter is very thin or the filling is too loose. Spread half the batter evenly and press the filling together a little with your fingers before scattering it.

Why can't there be any trays in the oven? A tray under the tin holds back the heat reaching the base. In a tall cake with as much fruit as this one, the centre would take much longer to set.

Will it work in a 2-litre tin? Only if you adjust the quantities: multiply every ingredient by 0.8 and start checking 10 minutes earlier.