For the Cupcakes:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
3. Cream Butter and Sugar: In a large bowl, beat the softened butter and granulated sugar with an electric mixer until light and fluffy, about 2-3 minutes.
4. Add Eggs and Vanilla: Beat in the eggs, one at a time, followed by the vanilla extract. Mix until well combined.
5. Alternate Adding Dry Ingredients and Milk: Add the dry ingredients in three batches, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
6. Bake: Divide the batter evenly among the cupcake liners, filling each about 2/3 full. Bake for 18-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.
7. Cool: Let the cupcakes cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

For the Custard Filling:

1. Heat the Milk: In a medium saucepan, heat the milk over medium heat until it begins to simmer.
2. Whisk Egg Yolks and Sugar: In a separate bowl, whisk together the egg yolks and sugar until pale and smooth.
3. Add Cornstarch: Whisk in the cornstarch until the mixture is smooth.
4. Temper the Eggs: Slowly pour the hot milk into the egg mixture while whisking constantly. This helps prevent the eggs from curdling.
5. Cook the Custard: Pour the egg mixture back into the saucepan and cook over medium heat, whisking constantly, until the mixture thickens (about 3-4 minutes).
6. Finish the Custard: Remove from heat and stir in the vanilla extract and butter until the butter melts and the custard is smooth.
7. Cool: Transfer the custard to a bowl, cover with plastic wrap (press the wrap directly onto the custard to prevent a skin from forming), and refrigerate until chilled.

For the Chocolate Ganache:

1. Heat the Cream: In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.
2. Make Ganache: Pour the hot cream over the chopped chocolate in a bowl. Let it sit for 2-3 minutes to soften, then stir until smooth and glossy.
3. Finish the Ganache: Stir in the butter until fully combined and the ganache is silky.

Assembling the Cupcakes:

1. Core the Cupcakes: Once the cupcakes are completely cool, use a small knife or cupcake corer to remove the center of each cupcake, creating a well for the custard filling.
2. Fill the Cupcakes: Spoon or pipe the chilled custard into each cupcake, filling it to the top.
3. Top with Ganache: Spoon or pour the chocolate ganache over the top of each filled cupcake, letting it drizzle down the sides.
4. Chill: Place the cupcakes in the refrigerator for about 15-20 minutes to allow the ganache to set before serving.