For the Vanilla Cupcakes:

1. Preheat your oven to 350°F (175°C) and line a muffin tin with paper cupcake liners.
2. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
3. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.
4. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
5. Gradually add the dry ingredients to the wet mixture, alternating with the milk. Begin and end with the dry ingredients, mixing until just combined.
6. Divide the batter evenly among the cupcake liners, filling each about 2/3 full.
7. Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
8. Allow the cupcakes to cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely.

For the Pastry Cream Filling:

1. Heat the milk and sugar in a medium saucepan over medium heat until it just begins to simmer. Do not boil.
2. Whisk the egg yolks and cornstarch together until smooth and pale in a separate bowl.
3. Gradually add the hot milk mixture to the egg yolk mixture, whisking constantly to prevent curdling.
4. Pour the egg mixture back into the saucepan and cook over medium heat, whisking continuously until thickened and coating the back of a spoon (about 3-5 minutes).
5. Remove from heat and stir in the butter and vanilla extract until smooth.
6. Transfer the pastry cream to a bowl, cover with plastic wrap (pressing it onto the surface), and refrigerate for 2 hours.

For the Chocolate Ganache Topping:

1. Heat the heavy cream in a small saucepan over medium heat until it begins to simmer.
2. Remove from heat, add the chocolate chips, and let sit for 2-3 minutes.
3. Stir until smooth and glossy, then let cool slightly before using to top the cupcakes.

Assembling the Cupcakes:

1. Once the cupcakes have cooled completely, use a knife or cupcake corer to make a hole in the center of each cupcake.
2. Fill each hole with chilled pastry cream.
3. Spoon or pipe the chocolate ganache over the top, allowing it to drizzle down the sides.
4. Let the ganache set before serving.