Preheat the oven to 400°F Melt the butter. Combine chopped chocolate with 2/3 cup plus 1 tbsp of sugar then add hot melted butter. Stir a few times, then let stand 3-4 minutes to melt chocolate. Stir until smooth.
Beat egg yolks in a large bowl, then slowly whisk in chocolate mixture, then add breadcrumbs. Stir until well-combined.
Beat egg whites and salt, either in a stand mixer with the whisk attachment, or with a hand-held electric mixer, at medium speed. When foamy, reduce speed to low and gradually add remaining 1/3 cup sugar. Increase speed back to medium and whip until stiff, shiny peaks form; do not overwhip. Fold egg whites into chocolate mixture in three or four equal portions.
Line 12 muffin cups with paper liners. Fill each liner about 3/4 full. Bake in preheated oven until puffed and just set in the center, about 10 minutes. Remove from oven and transfer to a rack until completely cool. Store in an airtight container at room temperature for up to 3 days.