Place a rack in the center of the oven and preheat it to 375 degrees. Place muffin cups into a 12-inch muffin tin and place on a baking sheet.
In a medium bowl placed over a small pan of simmering water, melt the butter and 1/2 of the chopped chocolate. Remove from heat.
Whisk together the flour, sugar, cocoa, baking powder, baking soda, and salt in a large bowl.
In a medium bowl, mix together the buttermilk, egg, and vanilla. Mix until combined.
Add the buttermilk mixture and the chocolate mixture to the dry ingredients. Stir quickly and gently with a rubber spatula being careful not to overmix.
Fold in the remaining chopped chocolate. Fill the muffin cups 2/3 full.
Bake for 20 minutes until a cake tester is inserted and comes out clean. Allow the pan to cool on a wire rack for 5 minutes before removing the muffins from the tin.
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