Preheat oven to 350°F (180°C). Line 12 muffin cups with liners.
In a heatproof bowl, melt chocolate and butter in the microwave in 20-second intervals, stirring until smooth.
In a small bowl, sift together flour, cocoa powder, and salt.
Whisk eggs and sugar until thick and pale, then fold in melted chocolate mixture. Gently fold in dry ingredients just until combined.
Divide batter evenly into muffin liners, filling about ¾ full. Top with chopped chocolate if desired. Bake for 13-15 minutes, then cool on a wire rack.
Notes
Be careful not to overbake to keep muffins moist and fudgy.