Preheat oven to 350F/180C. 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 a mixer fitted with the whisk attachment, 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 between liners, filling them about ¾ full. Bake for 13-16 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.
Store muffins in an airtight container at room temperature or in the refrigerator for up to 2 days. Allow to reach room temperature before serving.