Preheat oven to 180ºC and line a 20 x 30cm slice tin with baking paper.
Chop almonds, hazelnuts and baking powder in the Thermomix bowl 8 sec/speed 8. Set aside.
Chop chocolate in Thermomix bowl 5 sec/speed 8.
Add butter and sugar to Thermomix bowl and melt 3 min/90ºC/speed 2.
Add eggs, vanilla and reserved nut mixture and mix 30 sec/speed 4.
Pour into lined tin and bake 20-25 minutes, or until a skewer inserted into the centre comes out clean. Allow to cool in the tin.
When cool, slice into 16 to 20 pieces and dust with icing sugar.
Keeps 3-4 days.