Preheat your oven to 350°F. Line a cupcake pan with 4 cupcake liners.
In a small, microwave-safe bowl, combine butter, chopped chocolate, and vegetable oil. Microwave for 30 seconds, and stir until smooth, microwaving for an additional 15 seconds if necessary. Set aside to cool slightly.
In a small bowl, whisk together flour, cocoa powder, baking powder, and salt. Set aside.
In a medium bowl, whisk together sugar, egg yolk, and vanilla. Whisk in slightly cooled chocolate mixture until smooth.
Whisk in flour mixture until just combined. Add the milk and coffee and whisk until smooth. The batter will look thin.
Spoon or pour batter into prepared cupcake cups, filling 2/3 of the way full. Bake for 16 to 19 minutes, until a toothpick inserted in the center comes out clean or with only a few dry crumbs.
Cool cupcakes in pan for 5 minutes before transferring to a cooling rack to cool completely, 15 to 20 minutes.