Preheat oven to 350°F (180°C). Line 12 muffin cups with liners. Set aside.
In a medium heatproof bowl, combine chocolate and butter. Melt in the microwave in 20-second intervals, stirring after each until smooth. Alternatively, melt over a saucepan of simmering water, stirring occasionally.
Sift flour, cocoa, and salt into a small bowl and set aside.
In a separate bowl, use an electric mixer on high speed to whisk eggs and sugar until thick and pale, about 4 minutes. Reduce speed to low and gradually mix in the melted chocolate.
Gently fold in the flour mixture using a rubber spatula until just combined.
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full. Sprinkle with additional chopped chocolate if desired.
Bake for 13-15 minutes, until a toothpick inserted into the center comes out with moist crumbs. Do not overbake.
Let cool in the pan on a wire rack for 10 minutes, then remove from pan and let cool completely on the rack.