This dough is thick.
Chilling the dough is mandatory in order to avoid spreading.
Due to the issue some individuals had with dryness of the dough, I've reduced the amount of flour to 1 1/2 cups from 2 cups. Please make sure your butter and egg are at room temperature. Butter needs to be very soft. Don't add milk or more butter, just start adding flour and cocoa gradually and check the consistency.Once you start rolling cookies (after you let the dough rest and chill), dough will become very soft.
Do not over bake cookies, because they will get hard.