Preheat oven to 350℉/180℃. Line 12 muffin cups with liners. Set aside.

To make the brownies:

In a medium heatproof bowl, place chocolate and butter. Heat mixture in the microwave in 20-second intervals, stirring between each interval, until melted. Alternatively, you can set the bowl over a saucepan of simmering water, stirring occasionally. Set aside.

In a small bowl sift flour, cocoa, and salt. Set aside.

Using an electric mixer fitted with the whisk attachment or a handheld mixer, whisk eggs and sugar on high speed until thick and pale, about 4 minutes. Reduce speed to low and slowly add in chocolate mixture. Stop the mixer. Add flour mixture and fold, using a rubber spatula, just until combined. Do not overmix.

Divide batter evenly using a silicon muffin pan or muffin tin with muffin liners, filling them about ¾ full. Sprinkle extra chopped chocolate on top prior to baking, if desired. Bake for 13-15 minutes or until a toothpick inserted into the center of the muffins comes out with moist crumbs and not dry. Be careful not to overbake as this will make the brownies dry. Transfer to a wire rack and let cool for 10 minutes before removing from pan. Allow muffins to cool completely on a wire rack.