Preheat oven to 350F. Add eggs and sugar to a large mixing bowl and mix with an electric mixer. Mix until light and fluffy - about 3 minutes.
Chop chocolate bar into chunks and melt with butter in the microwave or over a double boiler. Only warm until the butter is just melted. Mix until evenly combined and pour chocolate and butter mixture into the eggs and sugar along with the vanilla extract. Mix again with the electric mixer.
Add cocoa powder, flour, and salt to the bowl and fold in with a spatula. Mix until the dry ingredients are just folded in.
Grease a mini muffin pan (or use muffin lines) and add 2 tbsp of batter to each of 24 muffin slots. Place in the oven and bake at 350F for 15-17 minutes. Remove from the oven and allow to cool COMPLETELY before removing from the muffin pan. Removing from the pan prior to cooling may make them crumble.
Notes
Substitute the chocolate bar with semi sweet chocolate chips
Make these mini brownies gluten free by using a gluten free flour blend in place of all purpose flour