Melt 1 stick of butter (1/2 cup) in the microwave. Combine melted butter and cocoa powder in a large mixing bowl. Stir until smooth.
Mix brown sugar and white sugar with the butter/cocoa mixture.
Add eggs and vanilla.
Mix in salt, baking soda, and flour.
Chill dough for 60 minutes.Preheat the oven to 350 degrees. Scoop dough onto parchment paper lined cookie sheets using a medium cookie scoop (2 Tbsp. size).
Bake cookies for 9 minutes.
While cookies bake, unwrap peanut butter cups and set them to the side.Leave cookies on the baking sheet instead of moving them to a cooling rack. Place a peanut butter cup onto each cookie as soon as it comes out of the oven. Let the cookies stand for 5 minutes (or until the peanut butter cups start to look glossy).
Add candy eyes. Since the peanut butter cups are glossy and soft the eyes will stick right to them.
Refrigerate the tray of cookies for 15 minutes (this will prevent the peanut butter cups from melting too much).
Melt chocolate chips in a microwave-safe bowl. Spoon chocolate into a frosting bag or sandwich bag. Use a frosting tip or cut the tip off of the bag (making a TINY hole) and pipe the chocolate onto the cookies to add hair and a mouth.