In a large bowl, cream together butter and sugars. Add eggs and vanilla extract. Beat until creamed and fluffy, about a minute.
In a separate bowl, whisk (or sift) together the flour, cocoa powder, salt, and baking soda.
Stir half of dry ingredients into butter mixture and stir until combined. Add milk and stir to combine. Add remaining dry ingredients and stir until just incorporated.
Cover bowl or wrap dough in wax paper. Chill for at 1 hour (up to 24 hours) in the refrigerator. Refrigerating will help the flavors meld and also create a firmer dough to roll into balls.
Preheat oven to 350 degrees.
Remove dough from refrigerator. Scoop dough using a 1.5 tablespoon scoop or shape by hand into 1.5 inch balls. Roll balls in sprinkles and place onto a parchment lined baking sheet. Decorate with candy eyeballs and m&ms candies (or other decorations you have on hand).
Bake for 10-12 minutes, or until cookies begin to crack on top. Allow to cool for 5 minutes before removing to a wire rack to cool completely.
Your cookies will keep for 5-7 days in a sealed container. To help keep cookies fresh, you can place a slice of bread in the container with the cookies. Replace the bread slice as needed.
You can keep raw cookie dough in the refrigerator for 3 days. Cookie dough balls (and baked cookies) freeze well, up to 3 months in a sealed plastic container or freezer bag. Allow frozen balls to thaw for up to 10 minutes on the baking sheet before baking, or add 1-2 minutes to the baking time.
Butterscotch chips can be substituted with white chocolate, semi-sweet chocolate, or peanut butter chips.