Cream the butter and sugar together just until well incorporated. It does NOT need to be fluffy.
Mix in the eggs and vanilla.
Add the dry ingredients (flour, cocoa powder, and salt) and mix until everything is incorporated. You should have a dense dough that clumps together.
Pat the dough in to a disc shape, wrap in plastic wrap, and refrigerate for at least 1 hour.
Heat oven to 350°F.
Roll out the dough on a dry surface (floured, if needed) until it's about 1/4 inch thick. Cut out cookies and place on a ungreased sheet pan about an inch apart. You can put the cut out cookie dough in the fridge again until it's extra firm (about 10-15 minutes) before baking if you want to make the edges as neat as possible, but it's not necessary.
Bake for 8--12 minutes or until the cookies are firm in their centers.