1. Make the Cupcakes

1. Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
3. In a large bowl, beat the softened butter and sugar with a hand or stand mixer until light and fluffy, about 2-3 minutes.
4. Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
5. Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix 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. Let the cupcakes cool completely on a wire rack.

2. Prepare the Custard Filling

1. In a medium saucepan, heat the milk over medium heat until it begins to steam (do not boil).
2. In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth.
3. Slowly pour the hot milk into the egg mixture, whisking constantly to temper the eggs.
4. Return the mixture to the saucepan and cook over medium heat, stirring constantly, until the custard thickens and coats the back of a spoon, about 2-3 minutes.
5. Remove from heat and stir in the vanilla extract. Transfer the custard to a bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and refrigerate until fully chilled.

3. Make the Chocolate Ganache

1. In a heatproof bowl, add the chocolate chips.
2. Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer.
3. Pour the hot cream over the chocolate chips and let sit for 1-2 minutes.
4. Stir until the chocolate is fully melted and the ganache is smooth and shiny. Add the butter for extra shine, if desired. Let the ganache cool slightly.

4. Assemble the Cupcakes

1. Using a small knife or cupcake corer, cut a small circle from the center of each cupcake, creating a hole about 1 inch deep. Save the cut-out pieces.
2. Spoon or pipe the chilled custard into each hole, filling it to the top. Replace the cut-out pieces to seal the cupcakes.
3. Spoon or dip the tops of the cupcakes in the chocolate ganache, letting it drip slightly down the sides for a beautiful finish.
4. Let the ganache set for about 10 minutes, then serve or refrigerate until ready to enjoy.