Position an oven rack in the center of the oven. Preheat the oven to 375°F. Butter or spray muffin top pan.

Melt the butter and half of the chocolate together in a bowl over a saucepan of simmering water. Once melted, remove the bowl from the heat and set aside. Alternatively, you can melt the butter and chocolate in a bowl in the microwave in 30-second increments until they are melted.

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. Fold the remaining chopped chocolate into the batter.

Dived the batter evenly among the muffins cups. Bake for about 20 to 25 minutes, or until a knife inserted into the center of the muffin tops comes out clean. Transfer the pan to a rack and cool.