First of all, leave butter for a few hours in room temperature, so it becomes soft. Chop fine walnuts and crush biscuits.
Then pour into a pot, sugar, cacao, milk. Medium heat pot and always stirring, heat up. When done, turn off the heat, add butter and mix well until butter melts. If needed, heat a little bit (but don't boil butter). Add crushed biscuits and walnuts, mix very well.
Let the mass cool down for a little. Then pour it onto a cling-film covered surface and form into a sausage shape, make sure everything is pressed together very well. Place in the refrigerator for a few hours, preferably overnight. Once it is solid and holds its shape – slice and enjoy!