Divide the dough into two parts.
In one of them add cocoa and sour cream.
Knead the chocolate dough.
Both types of dough wrapped in plastic wrap and send to the refrigerator for half an hour.
Divide each dough into 4 parts.
Roll out each part of the dough into a thin rectangle measuring 9*15 cm, about 2 mm thick. Trim the edges evenly, keep the trimmings.
Now spread alternately on each other the dough is white and chocolate, sprinkling each layer of dough coarsely chopped nuts.
From the saved scraps too to form layers.
The last layer of dough with walnuts sprinkle.
Wrap everything in parchment paper and roll a little with a rolling pin, pressing the layers. Send to the freezer for 30 minutes.
Then remove the parchment and cut the dough into pieces 1-1. 5 cm thick.
Put on a baking sheet covered with parchment and bake in a preheated 170 degree oven for 15-20 minutes.