Preheat oven to 350 F.
Chop chocolate bar in small pieces. I used Lindt chocolate bar with 70% cocoa, and it worked perfectly in this recipe.
Add finely chopped chocolate to the small bowl with hot coffee, stir well and let the mixture sit until all chocolate melts.
In a large bowl, add sugar, flour, cocoa powder, baking soda, and baking powder, and mix all these ingredients well.
In a separate large bowl, beat 3 eggs until eggs are fluffy and thickened, and slight foam forms – about 3 minutes by hand-held mixer.
Add oil, buttermilk, vanilla, and melted chocolate coffee mixture – continue beating until all are well incorporated.
Add flour-cocoa-sugar mixture to the wet ingredients and mix with the mixer just until everything is mixed well. Do not over-mix.
Line cupcake pans or muffin tins with liners.
Pour batter into lined muffin tins or cupcake pans - so that each cupcake is about ⅔ full. Don’t fill them all the way to the top, because the dough will rise, as it’s baking.
Bake cupcakes for 20-30 minutes, until tooth pick inserted in the middle of the single cupcake comes out clean.