Preheat oven to 425 degrees Fahrenheit. Line muffin tin with cupcake liner.
Chop the chocolate bar into small pieces. Put the chocolate bar and butter in a microwavable bowl and microwave for about 1 minute. Stir with a wooden spoon until it becomes a silky mixture.
Add the sugar, eggs, buttermilk and vanilla. Mix together until thoroughly mixed. This is the wet mixture.
In a large bowl, stir flour, cocoa powder, baking soda and salt. Make a well in the center and slowly pour half of the wet mixture. Fold and then add the other half of the wet mixture. Fold all the way to the bottom of the bowl, until the batter becomes thick and evenly mixed.
Add the chocolate chips and fold again.
Using a large ice cream scoop, scoop batter and fill the muffin liner all the way to the top. Sprinkle the top with about 3-4 mini chocolate chips.
Bake at 425 degrees Fahrenheit for 5 minutes. Turn down the oven temperature to 350 and bake for another 15-17 minutes. Check doneness by inserting a toothpick in the middle of the muffin. The toothpick should come out clean and dry.
Cool muffins in the muffin tin for 5 minutes. Transfer to a wire rack to cool completely.