Make the Cupcakes:
Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners. In a bowl, whisk together flour, baking powder, baking soda, and salt. In another bowl, cream the butter and sugar until light and fluffy. Beat in the eggs one at a time, followed by the vanilla. Gradually add the dry ingredients and milk, mixing until just combined. Fill the cupcake liners ¾ full and bake for 18-20 minutes or until golden. Let them cool completely.

Prepare the Pastry Cream:
Heat milk in a saucepan over medium heat until warm but not boiling. In a separate bowl, whisk egg yolks, sugar, and cornstarch. Slowly pour the warm milk into the egg mixture while whisking constantly. Return the mixture to the saucepan and cook over medium heat, stirring continuously, until thickened. Remove from heat, stir in vanilla and butter, then transfer to a bowl. Cover with plastic wrap, pressing it directly onto the surface, and chill completely.

Fill the Cupcakes:
Using a small knife or piping tip, cut a hole in the center of each cupcake. Spoon or pipe in the chilled pastry cream, filling each cupcake generously.

Make the Chocolate Ganache:
Place chopped chocolate in a bowl. Heat the heavy cream until steaming, then pour it over the chocolate. Let sit for a minute, then stir until smooth. Stir in butter for extra shine.

Assemble the Cupcakes:
Dip or spoon the ganache over the tops of the filled cupcakes. Let the ganache set before serving. Enjoy!