Preheat the oven to 165-175 °C hot air.
Beat sugar, yolks and water until foamy. Sift flour with baking powder and beat egg whites until stiff. Mix everything with the nuts and bake the dough for about 40 minutes.
For the cream, mix cornstarch with some milk, bring milk with vanilla to a boil and stir in cornstarch. Let everything cool down.
Cream butter and sugar and stir in cold starch mixture. Spread the cream on the nut cake and serve the cake.