For the Cupcakes:

1. Preheat oven to 350°F (175°C) and line a cupcake pan with liners.
2. In a medium bowl, whisk together the flour, baking powder, and salt.
3. In a large bowl, beat the butter and sugar until light and fluffy (about 2–3 minutes).
4. Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
5. Alternately add the dry ingredients and milk, beginning and ending with the dry ingredients. Mix until just combined.
6. Divide the batter evenly among cupcake liners, filling each about ⅔ full.
7. Bake for 18–20 minutes or until a toothpick inserted into the center comes out clean.
8. Let cupcakes cool completely before filling.

For the Pastry Cream Filling:

1. Heat the milk in a small saucepan over medium heat until steaming (not boiling).
2. In a separate bowl, whisk together egg yolks, sugar, and cornstarch until smooth.
3. Slowly pour half of the hot milk into the egg mixture, whisking constantly to prevent curdling.
4. Return mixture to the saucepan and cook over medium heat, whisking constantly, until thickened (about 2 minutes).
5. Remove from heat and stir in vanilla extract and butter. Let cool completely.

For the Chocolate Ganache:

1. Heat the heavy cream in a saucepan or microwave until hot (not boiling).
2. Pour over the chopped chocolate and let sit for 2 minutes, then stir until smooth.
3. Stir in the butter for extra shine. Let cool slightly before using.

Assembly:

1. Fill the Cupcakes: Use a knife or piping tip to hollow out the center of each cupcake. Pipe or spoon the cooled pastry cream inside.
2. Top with Ganache: Spoon or dip each cupcake into the chocolate ganache, letting the excess drip off.
3. Optional Buttercream: Pipe a swirl of vanilla buttercream over the ganache for an extra decorative touch.