Preheat oven to 350F and line a 12-cup muffin pan with paper wrappers. Set aside.
Place butter and chocolate in a large bowl and microwave 60-90 seconds, stirring at 30 second intervals until chocolate is melted. Allow to cool slightly.
Whisk sugars, eggs, vanilla, flour, cocoa powder, baking powder, and salt into cooled chocolate mixture until smooth. Spoon batter into prepared muffin cups, filling 3/4 full.
Bake cupcakes at 350F 20-25 minutes until a toothpick inserted in center comes out clean. Cool cupcakes in pan completely on a wire cooling rack.