Center a rack in the oven and preheat the oven to 375 degrees F.
Butter or spray the 12 molds in a regular-size muffin pan or fit the molds with paper muffin cups (Alternatively, use a silicone muffin pan, which needs neither greasing nor paper cups.)
Place the muffin pan on a baking sheet.
Melt the butter and half the chopped chocolate together in a bowl over a saucepan of simmering water; or do this in a microwave.
Remove from the heat.
In a large bowl, whisk together the flour, sugar, cocoa, baking powder, baking soda and salt.
In a large glass measuring cup or another bowl, whisk the buttermilk, egg and vanilla extract together until well combined.
Pour the liquid ingredients and the melted butter and chocolate over the dry ingredients and, with the whisk or a rubber spatula, gently but quickly stir to blend.
Don?t worry about being thorough ? a few lumps are better than overmixing the batter.
Stir in the remaining chopped chocolate.
Divide the batter evenly among the muffin molds.
Sprinkle tops with coarse white sugar.
Bake for 20 minutes, or until a thin knife inserted into the center of the muffins comes out clean.
Transfer the pan to a rack and cool 5 minutes before carefully removing each muffin from its mold.
