Preheat oven to 350°F.
In a mixing bowl, cream together the butter and brown sugar.
Add the egg and vanilla, mixing thoroughly.
In another bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Mix the dry into the wet and stir to combine.
Fold in your mix-in chips.
Pour and pile the dough onto your baking mat, and flatten and shape into a 3" disc. Give it room to spread out as it bakes!
Bake for 15-16 minutes. The cookie will look soft when it's done, but will firm up as it cools. Bake slightly longer for a crispier cookie, but be careful not to let the edges burn.
Let cool until you want to enjoy!