Preaheat oven to 350F degrees. Grease and flour a non stick mini muffin/cupcake pan.
Place the chopped chocolate and butter In a medium microwave safe bowl and melt in the microwave on full power in 20-30 second blasts, stirring after each one until smooth.
Whisk cocoa powder into the butter mixture until smooth.
Add sugar, brown sugar, eggs, salt and vanilla to the mixture and whisk until smooth.
Vigorously stir the flour into the mixture until the batter is completely smooth and no streaks or lumps of flour remain. Do not worry about over-mixing for this recipe.
Scoop the batter into the prepared mini muffin pan, filling each about 3/4 full. If you have a 24-cup pan, you may have some batter left to make a few more after the first batch is done. I usually get around 30 mini brownies.
Bake in the preheated oven on the middle rack for about 11-13 minutes. Brownies are done when a toothpick or cake tester comes out with a few moist crumbs clinging to it. Be sure not to overbake, or they will be dry.
Let cool in the pan for a few minutes and then remove to cool completely. If you have any trouble taking them out, run a knife along the inside edge to loosen them first. I find a gentle twisting motion helps them come out easily.