Heat the oven to 160°C. Tip the hazelnuts onto a baking tray and slide into the oven to toast for 15-20 mins, they should be golden brown. Set aside to cool completely.

Grease a 23cm cake tin with butter, then cut a circle the same size as the base and use it to line the base of the tin. Grease again with butter, then tip cocoa powder into the tin and shake it around so all the sides are covered.

Cut the butter into 2-3cm cubes and add to a glass bowl. Roughly chop the chocolate and add it to the bowl. Either melt them together in short bursts in the microwave or in a glass bowl over a pan of simmering water. Once silky and smooth, whisk in the cocoa powder and vanilla extract and set aside.

Separate the eggs into two bowls. Use an electric hand whisk to whip egg whites until they stand in soft peaks when you lift the beaters up. Add the caster sugar and whip until thick and glossy.

Move to the bowl of yolks. Add the brown sugar and whip until thick and pale. You don’t need to wash the beaters in between. Add the chocolate mixture to the yolks and stir until combined.

Add a spoonful of the egg white and use the beaters to whip it in, then use a large spoon or spatula to fold in the rest of the egg white, be careful not to knock out too much air out of the mix, you want it to be combined but still voluminous.

Tip ¾ of the hazelnuts into a blender and blitz until floury. Careful not to blend for too long or they’ll start to release oils and become like a butter. You want them to be finely blitzed but not to become oily – I do this by blitzing them in small bursts. Fold the hazelnuts into the chocolate mix, then spoon into the prepared cake tin.

Transfer the cake into the oven for 25 mins, then remove from the oven and set aside for at least 20 mins to sink. The longer you leave it, the easier it’ll be to slice.

When ready to serve, crush the remaining hazelnuts. Slice the cake, then serve each slice with a dollop of crème fraîche and a scattering of crushed hazelnuts.