Preheat your oven to 350F (175C) and line 2 12-count muffin tins with paper liners. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, whisk together flour, sugars, cocoa powder, baking soda, and salt.
Add melted butter and oil, stir well.
Add eggs and egg yolk, one at a time, stirring well after each addition. Pause occasionally to scrape down the sides and bottom of bowl.
Stir in vanilla extract. Gradually add buttermilk and stir well, then stir in chopped chocolate.
Add hot coffee, stirring until ingredients are well-combined (be sure to scrape sides and bottom of bowl again).
Evenly divide batter into prepared muffin tins. Do not fill muffin tins higher than ⅔ of the way full or the batter will spread out over the top of the pan and you'll have awkward and unappealing flat-topped cupcakes.
Bake on 350F (175C) for 18 minutes or until a toothpick inserted in center should come out with moist crumbs (not wet batter).
Allow to cool completely in the muffin tins before removing and decorating with your favorite frosting. I recommend using my peanut butter frosting.
*If you don't have hot coffee, hot water can be used.