Make the Cupcakes:

1. Prepare the Batter:
   • 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 the eggs, one at a time, then add the vanilla. Alternate adding the dry ingredients and milk, beginning and ending with the dry ingredients. Mix until just combined.
2. Bake the Cupcakes:
   • Divide the batter evenly among the cupcake liners. Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean. Let the cupcakes cool completely.

Make the Custard Filling:

3. Cook the Custard:
   • In a saucepan, heat the milk over medium heat until it begins to steam (do not boil).
   • In a separate bowl, whisk together the egg yolks, sugar, and cornstarch. Gradually whisk the hot milk into the egg mixture to temper it. Return the mixture to the saucepan and cook over medium heat, stirring constantly, until thickened (about 2-3 minutes). Remove from heat and stir in the vanilla.
   • Transfer the custard to a bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin), and refrigerate until chilled.

Make the Chocolate Ganache:

4. Prepare the Ganache:
   • Heat the heavy cream in a small saucepan or microwave until it begins to simmer. Pour the hot cream over the chopped chocolate in a bowl. Let sit for 1 minute, then stir until smooth.

Assemble the Cupcakes:

5. Fill the Cupcakes:
   • Use a knife or cupcake corer to remove the center of each cupcake. Spoon or pipe the chilled custard into the holes. Replace a small piece of the removed cupcake to cap the filling if desired.
6. Top with Ganache:
   • Spoon or dip each cupcake in the chocolate ganache, allowing it to drip slightly down the sides. Let the ganache set for about 10 minutes before serving.