Chocolate Chunk Muffins

Center a rack in the oven and preheat the oven to 375°F. Butter or spray the 12 molds in a regular-size muffin pan or fit the molds with paper muffin cups. Place the muffin pan on a baking sheet.
Melt the butter and half of the chocolate together in a bowl over a saucepan of simmering water; or do this in a microwave. Remove from the heat.
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 through - a few lumps are better than overmixing the batter. Stir in the remaining chopped chocolate. Dived the batter evenly among the muffins cups.
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.
Bake for about 20 minutes, or until a think knife inserted into the center of the muffins comes out clean. Transfer the pan to a rack and cool for 5 minutes before carefully removing each muffin from its mold.