Before you start, leave the butter to soften in room temperature. Grind the cookies. Heat sugar, cacao and milk, stirring in a pot until hot. Turn of the heat, add butter and mix well. If the butter didn't melt, heat some more, but don't bring to boil. Then add the cookies and mix well. After the mixture has cooled down, wrap it tightly, shaping the cake and leave in the fridge for several hours.