Preheat the oven to 160°C. Into a mixing bowl, sift together the flour and baking powder, then fold in the ground hazelnuts.
Into a separate mixing bowl, whisk the eggs and sugar until very pale and fluffy, then gradually add the melted butter continuing to whisk until well combined. Now fold in the flour and hazelnut mixture adding the zest and juice of the lemon. Gently fold the raspberries through the mixture.
In the meantime, butter a 23cm loose bottomed cake tin, lining the bottom with baking paper, also greased. Pour the cake mixture into the tin, place into the oven and bake for 40 minutes.
Remove from the oven and allow to cool in the tin for 20-25 minutes, then turn out on to a wire rack to cool thoroughly. The cake should be slightly undercooked in the centre as the cooling process will help to finish it off.
Serve with either sour cream or crème fraiche and some more raspberries.
This recipe appears in the Celebrity Bake Book, in aid of the Ben Kinsella Trust.