Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.

In a medium bowl, whisk together the flour, baking powder, and salt.

In a large bowl, cream the butter and sugar until light and fluffy. Beat in eggs one at a time, then mix in vanilla.

Add dry ingredients to the butter mixture alternately with milk, mixing just until combined.

Divide the batter evenly among cupcake liners and bake for 18–20 minutes or until a toothpick comes out clean. Cool completely.

For the custard, heat milk until steaming. In a separate bowl, whisk egg yolks, sugar, and cornstarch. Gradually whisk in hot milk to temper, then return to saucepan and cook until thickened. Stir in vanilla and chill with plastic wrap pressed onto the surface.

To make ganache, heat cream until simmering. Pour over chocolate, let sit 1 minute, then stir until smooth.

Use a knife or cupcake corer to remove the center of each cupcake. Fill with custard using a spoon or piping bag. Cap with a piece of the removed cupcake if desired.

Top each cupcake with ganache and let set 10 minutes before serving.