Preheat oven to 350 degrees. Grease a 9 x 13 baking pan with butter and set aside.
In a medium pan, melt butter over medium-low heat.
When butter is completely melted, use a rubber spatula to mix in cocoa powder.
Turn off heat and stir in vanilla and sugar.
Add eggs one at a time, mixing well after each addition.
Stir in flour, baking powder and salt until fully incorporated.
If you are adding chocolate chips and/or chopped nuts, gently mix them in now.
Pour batter into the prepared 9 x 13 baking dish. Bake for 20-25 minutes until toothpick inserted in the center comes out with only a few crumbs on it. It will finish the baking process as it cools.
Notes
You can add any flavor chips to these brownies just before baking - milk chocolate, semi-sweet, peanut butter or white chocolate.
Brownies cut easier and cleaner when they are fully cooled -- if you can wait that long!
The brownies will taste better a couple of hours after they come out of the oven, because the flavors have had time to bind together during the cooling process.