Butter 9x9 baking pan and preheat your oven to 350 degrees.
Cream together butter and sugar. When well incorporated, switch from a whisk to a wooden spoon or rubber spatula. Mix in vanilla and salt.
Add eggs one at a time, stirring well after each.
Thoroughly mix in cocoa powder then add in flour half at a time. Do not over mix!
Fold in your mix-ins. I usually use butterscotch chips or white chocolate chips. Transfer mixture to buttered baking pan and bake for 25 minutes (or until a toothpick stuck in the center comes out mostly clean) at 350 degrees. I would advise checking them at 20 minutes as every oven is slightly different.