Preheat oven to 350°F (175°C).
Grease a muffin tin to prevent sticking.
Whisk melted butter, granulated sugar, and brown sugar until smooth.
Add eggs and vanilla, mixing until fully combined.
Sift in flour, cocoa powder, salt, and baking powder.
Fold the dry ingredients into the wet mixture.
Scoop batter into the muffin tin, filling each cup halfway.
Bake for 15-18 minutes or until the edges set.
Use a spoon to press a well into each brownie while warm.
Let the brownie cups cool completely before filling.