Preheat your oven to 350°F. Grease a 9x9" baking dish with a bit of butter, and line the bottom with parchment paper. I like to leave enough to hang over the sides, to easily remove the brownies later.
Sift together your flour and cocoa powder, and set aside.
Combine your melted butter, brown sugar, and vanilla in a mixing bowl.
Add in the eggs one at a time, mixing them well each time.
Add in the cocoa and flour and mix well. Try not to over-mix, but mix until there are no more visible clumps of dry ingredients.
Fold in your mix-ins.
Spread the batter evenly on the bottom of your baking dish. Bake for 30-35 minutes, or until a toothpick inserted comes out clean. Baking longer will make the edges chewier.
Let cool for about 15 minutes, and you should be able to lift the brownies out using the parchment paper.