Preheat oven to 350°F. Grease two cookie sheets or line them with parchment paper.
Combine softened butter and sugars, beat with an electric mixer until light and fluffy, about 1-2 minutes.
Add eggs one at a time and continue to beat for about 1 more minute.
Add vanilla and mix.
In a separate bowl, combine flour, baking soda, salt, and cocoa. Whisk together until there are no lumps, then slowly add the dry mixture to the butter mixture, combine well but don’t overmix.
Form dough into two-tablespoon sized dough balls and place evenly spaced on cookie sheet.
Add any mix-ins that you choose. In these, the mix-in options I had was 1- a smashed Rolo, 2-green mint chips, 3-white chocolate peppermint chunks, 4-peanut butter chips, 5-chocolate chip,s and 6-one with no mix-ins, to which I added an Andes mint to the top of the hot cookie and let it melt, then spread it with a knife.
Bake cookies for 8-9 minutes, then let cool.