Heat oven to 180C/160C fan/gas 4. Butter two 20cm cake tins and line the bases, then dust well with flour and tap out the excess. Put the chocolate in a small bowl set over a pan of gently simmering water. Stir until melted, then set aside to cool.
Mix the espresso with 250ml boiling water, then leave to cool. Blitz the hazelnuts in a food processor until finely ground.
Combine the ground hazelnuts, sugar, flour, a good pinch of salt and the bicarb in a mixer. Mix the eggs, soured cream, vanilla, butter, hazelnut oil and the cooled espresso in a jug. With the mixer on a low speed, gradually add the egg mixture to the sugar mixture and combine until smooth. Add the melted chocolate and beat for 15 seconds.
Divide the batter between the tins (it’ll be quite thin at this stage) and bake for 35-40 mins, or until a skewer inserted into the middle of each cake comes out clean. Leave to cool in the tin for 10 mins, then run a knife around the edge and turn the cakes out onto a wire cooling rack. Leave to cool completely.
For the buttercream filling, dissolve the espresso powder in 1 tbsp boiling water and leave to cool. Beat the butter and icing sugar in a mixer or by hand in a bowl, gradually adding the cooled coffee. Use a palette knife to spread the buttercream over the top of one sponge.
To make the topping, gently heat the cream, espresso powder and sugar in a pan, stirring to help the sugar and coffee dissolve. Take off the heat and add the chocolate. Stir to melt until smooth and glossy, then leave to cool until thickened. Toast the whole hazelnuts in a dry frying pan, leave to cool, then chop very roughly (you’ll want some halves in the mixture).
Place the top layer of cake onto the bottom layer and cover with the glaze. Scatter the hazelnuts on top.