Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners and set aside.

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

In a large bowl, cream the softened butter until smooth using an electric mixer. Add the eggs, one at a time, beating well after each addition. Add the vanilla extract and mix until combined.

Gradually add the dry ingredients to the butter mixture, alternating with the milk, and mix until just combined.

Spoon the batter evenly into the prepared muffin tin, filling each cup about 2/3 full. Bake for 18-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

For the pastry cream: In a saucepan, heat the milk over medium heat until it begins to simmer. In a separate bowl, whisk together the sugar, egg yolks, and cornstarch until smooth. Gradually pour the hot milk into the egg mixture, whisking constantly. Return the mixture to the saucepan and cook over medium heat until it thickens, about 2-3 minutes. Remove from heat and stir in the butter and vanilla extract. Let it cool in the fridge for at least 2 hours.

For the ganache: Place the chopped chocolate in a bowl. Heat the heavy cream in a small saucepan until it simmers, then pour it over the chocolate and let it sit for 2-3 minutes. Stir until smooth and glossy, then mix in the butter. Let the ganache cool to room temperature.

Once the cupcakes have cooled, use a cupcake corer or a small knife to create a cavity in the center of each cupcake. Fill the cavity with the chilled pastry cream.

Spoon or pipe the ganache over the top of each cupcake. Let the ganache set before serving.