Lightly butter 2 × 20cm diameter, 4.5cm deep, sandwich tins, then line the bases with baking parchment.
Scatter the nuts for both the cake and the buttercream on to a baking sheet and
Sift together the flour and baking powder into a bowl and set aside.
In an electric mixer, or in a large bowl with an electric whisk, cream together the butter and sugar for about 5 minutes until very light and fluffy. Add the eggs very slowly, with the mixer or whisk running, adding
Divide the batter between the tins and
Meanwhile, make the buttercream. Cream the butter and sugar together for at least 5 minutes until really light and airy, then add the coffee. Mix well, then fold in some of the hazelnuts, keeping most back to decorate. Spread half the coffee buttercream on to the base of one cake, lay the other cake on top (flat bases together) and finish with the remaining buttercream. Scatter over the reserved hazelnuts.