To melt the chocolate, place in a double boiler over simmering water on low heat for 5-10 minutes (about; stir occasionally until completely smooth and no pieces remain; remove from heat and let cool 5-15 minutes or until lukewarm.
Line two 12-cup muffin tins with cupcake papers; set aside.
In a bowl, sift the flour and baking soda together; set aside.
In a big mixing bowl, cream the butter, using an electric mixer on MEDIUM speed, until smooth.
Add the sugars and beat for about 3 minutes or until fluffy.
Add the eggs, one at a time, beat well after each addition.
Add the chocolate, mixing until well incorporated.
Add the dry ingredients, in three parts, alternating with the buttermilk and vanilla.
With each addition, beat until ingredients are incorporated but do not overmix.
Scrape down the batter from the sides of the bowl, using a rubber spatula, to make sure the ingredients are well blended and the batter is smooth.
Carefully spoon batter into cupcake liners; fill 3/4 full.
Bake in a 350° oven for 20-25 minutes or until pick comes out clean.
Cool in tins for 15 minutes.
Remove from tins and cool completely on a wire rack.