Follow these simple steps to create your Boston Cream Pie Cupcakes:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a cupcake pan with paper liners.
2. Make the Cupcake Batter: In a large bowl, cream together the softened butter and sugar until light and fluffy.
3. Add Eggs and Vanilla: Beat in the eggs one at a time, then add the vanilla extract.
4. Combine Dry Ingredients: In another bowl, whisk together the flour, baking powder, and salt.
5. Mix Wet and Dry Ingredients: Gradually add the flour mixture to the butter mixture, alternating with the milk. Stir until just combined.
6. Fill the Cupcake Liners: Spoon the batter into the prepared cupcake liners, filling each about two-thirds full.
7. Bake: Bake in the preheated oven for 15-20 minutes, or until a toothpick inserted into the center comes out clean.
8. Cool the Cupcakes: Allow the cupcakes to cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
9. Prepare the Custard Filling: In a saucepan, heat the milk over medium heat until just simmering. In a separate bowl, whisk together the egg yolks, sugar, and cornstarch. Gradually whisk the hot milk into the egg mixture. Return it to the saucepan and cook over medium heat, stirring constantly until thickened. Remove from heat and stir in the butter and vanilla. Let it cool completely.
10. Prepare the Chocolate Ganache: In a small saucepan, heat the heavy cream until it just begins to simmer. Pour it over the chocolate chips in a bowl. Let it sit for a few minutes, then stir until smooth and glossy.
11. Fill the Cupcakes: Once the cupcakes have cooled, use a small knife or cupcake corer to remove a portion from the center of each cupcake. Fill each cupcake with the cooled custard filling.
12. Top with Ganache: Drizzle or spread the chocolate ganache over the filled cupcakes, allowing it to drip down the sides for an elegant look.