Preheat oven to 350°F (180°C). Line 12 muffin cups with liners. Set aside.
In a medium heatproof bowl, place chocolate and butter. Heat in the microwave in 20-second intervals, stirring between each, until melted. Alternatively, set the bowl over a saucepan of simmering water, stirring occasionally.
Sift flour, cocoa, and salt into a small bowl. Set aside.
Using an electric mixer 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. Fold in flour mixture using a rubber spatula, just until combined.
Divide batter evenly in the prepared muffin pan, filling about ¾ full. Sprinkle extra chopped chocolate on top before baking, if desired.
Bake for 13-15 minutes, or until a toothpick inserted comes out with moist crumbs. Do not overbake. Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.