Make the Cupcakes

1. Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.
2. In a large mixing bowl, cream together butter and sugar until light and fluffy.
3. Add vanilla extract and mix well.
4. Beat in eggs, one at a time, ensuring each is fully incorporated before adding the next.
5. In a separate bowl, whisk together flour, baking powder, and salt.
6. Alternate adding the flour mixture and buttermilk to the butter mixture, starting and ending with the dry ingredients. Mix until just combined.
7. Fill cupcake liners ¾ full with batter.
8. Bake for 18-25 minutes, or until a toothpick inserted into the center comes out clean.
9. Let cupcakes cool completely before filling.

Prepare the Custard Filling

1. In a medium bowl, whisk egg yolks. Gradually stir in milk.
2. In a saucepan, mix sugar, cornstarch, and salt.
3. Slowly add the milk mixture, stirring constantly to prevent lumps.
4. Cook over medium-low heat, stirring frequently until thickened and gently boiling (~20 minutes).
5. Remove from heat, stir in vanilla extract.
6. Cover custard with plastic wrap pressed directly onto the surface to prevent a skin from forming. Chill in the refrigerator until ready to use.

Make the Ganache

1. Place chopped chocolate in a heatproof bowl.
2. Heat heavy cream in a small saucepan over medium heat until it begins to steam (do not boil).
3. Pour hot cream over chocolate and let sit for 1-2 minutes.
4. Whisk until smooth and shiny.
5. Let ganache cool slightly until thick but still pourable.

Assemble the Cupcakes

1. Use a knife or cupcake corer to remove a small portion from the center of each cupcake.
2. Fill a piping bag with the custard and pipe it into the hollowed-out cupcakes.
3. Dip the tops of the cupcakes into the ganache, allowing excess to drip off.
4. Let the ganache set for 30-60 minutes before serving.