How to cook
Bake the walnuts briefly in the oven to make them more fragrant. Then let them cool. Mix them with the flour in a blender and blend until they turn to coarse crumbs.
Mix the chocolate and butter in a bowl, melt them in a water bath on a low stove. Stir until the mixture becomes smooth.
Separately, beat the yolks, 1/2 the sugar and vanilla for about 5 min. Beat the whites with the sour cream to a fluffy snow with the remaining sugar.
Carefully mix the chocolate mixture with the yolks, next stir with the whites slowly, then add the flour with nuts.
Pour the mixture into a 9 1/2″ (24 cm) diameter form and put it to bake in a preheated 356°F (180 °C) oven for 35 min. Let the cake cool before cutting it.
Optionally, you can pour melted chocolate over it or sprinkle it with some powdered sugar.