Preheat the oven to 350°Line your cupcake pan with 4 cupcake liners.
In a small, microwave-safe bowl, microwave the butter until it’s mostly melted. Add the semi-sweet chocolate and vegetable oil and microwave in 15-second bursts, stirring after each, until the chocolate is completely melted. Stir and set aside to cool.
In a small bowl, whisk together the flour, cocoa powder, baking powder, and salt.
In a medium bowl, whisk together the granulated sugar, egg yolk, and vanilla. Whisk in the cooled chocolate (if it is not cooled it will scramble the egg), until well combined.
Stir in the flour mixture, mixing just until combined.
Whisk in the milk and coffee until smooth. The batter will be thin. Pour the batter into the prepared tin, filling just under three-fourths full. If you have leftover batter, discard it.
Bake for 16 to 19 minutes until a toothpick inserted into the cupcake center comes out clean.
Cool the cupcakes in the tin for 10 minutes before transferring them to a cooling rack. Cool completely, for about 15 minutes, before making the buttercream.