Make the Cupcakes:
Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.
In a bowl, whisk together flour, baking powder, and salt.
In a separate bowl, beat butter and sugar until light and fluffy (about 2 minutes).
Add eggs and vanilla, mixing well.
Alternate adding dry ingredients and milk, mixing until combined.
Divide batter evenly into 12 cupcake liners (about ¾ full).
Bake for 18-20 minutes, until a toothpick inserted comes out clean.
Let cupcakes cool completely.
Make the Pastry Cream Filling:
In a saucepan, heat milk over medium heat until warm (but not boiling).
In a bowl, whisk egg yolks, sugar, and cornstarch. Slowly pour warm milk into the mixture while whisking.
Return mixture to the saucepan and cook over low heat, stirring constantly until thickened (about 3-4 minutes).
Remove from heat, stir in vanilla and butter, then transfer to a bowl.
Cover with plastic wrap (touching the surface) and chill for at least 30 minutes.
Fill the Cupcakes:
Using a small knife or piping tip, cut a hole in the center of each cupcake.
Fill each hole with chilled pastry cream using a spoon or piping bag.
Make the Chocolate Ganache:
Heat heavy cream in a saucepan until hot (but not boiling).
Pour over chopped chocolate and let sit for 2 minutes.
Stir until smooth and glossy.
Assemble And Serve:
Spoon or drizzle ganache over the filled cupcakes.
Let set for a few minutes before serving.