Preheat the Oven:
Preheat oven to 350°F (175°C) and line a muffin tin with paper liners.

Prepare the Cupcake Batter:
In a bowl, whisk together the flour, baking powder, and salt. In a separate bowl, beat the softened butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Combine the Mixtures:
Alternate adding the flour mixture and milk to the butter mixture, beginning and ending with the flour mixture. Mix just until combined.

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

Make the Pastry Cream:
In a saucepan, heat the milk until steaming. In a separate bowl, whisk together the sugar, cornstarch, salt, and egg yolks until smooth. Slowly add the hot milk, whisking constantly to prevent curdling.
Return the mixture to the saucepan and cook over medium heat, whisking until it thickens. Remove from heat, stir in butter and vanilla extract. Cover and chill in the refrigerator.

Prepare the Chocolate Ganache:
In a saucepan, heat the cream until just simmering. Pour over the finely chopped chocolate and let sit for 1 minute, then stir until smooth.

Assemble the Cupcakes:
Using a paring knife or cupcake corer, remove the center of each cupcake. Fill the center with the chilled pastry cream.

Top with Ganache:
Spoon the ganache over the top of each cupcake and allow it to set before serving.