Dice the cold butter from the fridge and add it to the flour. Cut it all in with a knife into fine crumbs.
Add the egg and sugar, and quickly knead the dough.
Divide the dough into 2 parts. Into one part, add the cocoa powder and sour cream and knead into a second, chocolate dough.
Shape each into a ball and refrigerate for 30 minutes.
Take out the dough and cut each ball into 4 pieces. Roll each piece into a rectangular sheet and stack the sheets on top of each other one by one, placing walnuts between the layers.
Wrap the assembled "block" in parchment paper, press firmly with your hands and shape it into a rectangle. Freeze for 30 minutes.
Take out the block, slice the cookies about 1 cm wide, place on a baking sheet and bake for 20-25 minutes at 180 °C (355 °F). Enjoy!