1.Preheat oven to 180°C/160°C fan-forced. Grease a deep 22cm-round cake pan; line base and sides with baking paper.
2.In a medium bowl, beat the butter and sugar with an electric mixer until light and fluffy; add the eggs, one at a time, beating until just combined between additions. (The mixture will curdle at this stage, but will come together later.)
3.Transfer mixture to a large bowl; stir in sifted flours, hazelnut meal and sour cream. Fold in raspberries and spread mixture into pan.
4.Bake about 1 hour 30 minutes. Stand cake in pan 10 minutes; turn, top-side up, onto wire rack to cool.
5.To make mascarpone crea: combine mascarpone, icing sugar, liqueur and sour cream in a medium bowl. Stir until smooth; stir in nuts.
6.Place cold cake on a serving plate.
7.Spread cake all over with mascarpone cream. Top and serve with extra fresh raspberries, if desired.
If using frozen raspberries, don’t thaw them; frozen berries are less likely to bleed into the cake mixture.Note