Preheat the oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners and lightly grease them. In a large mixing bowl, cream the butter and sugar using a hand or stand mixer on medium speed until light and fluffy, approximately 3 to 4 minutes. Add the vanilla extract and beat briefly to combine. Incorporate the eggs one at a time, ensuring each is fully mixed before adding the next.

In a separate bowl, sift together the flour, baking powder, and salt. Alternating with the buttermilk, add the dry ingredients to the wet mixture in three parts, beginning and ending with the flour. Mix until just combined; do not overmix.

Divide the batter evenly among the prepared cupcake liners, filling each approximately three-quarters full. Bake for 18 to 25 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool completely on a wire rack before filling.

In a medium bowl, whisk the egg yolks until smooth. Gradually add the milk and whisk until well combined. In a heavy-bottomed saucepan, mix the sugar, cornstarch, and salt. Slowly pour in the milk mixture, whisking constantly to avoid clumping.

Cook the mixture over medium-low heat, stirring continuously, until it thickens and reaches a gentle boil—approximately 15 to 20 minutes. Immediately remove from heat and stir in the vanilla extract. Transfer the custard to a heatproof bowl, cover the surface with plastic wrap to prevent skin formation, and refrigerate for at least 1 hour or until well chilled.

Place the chopped chocolate into a heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it begins to simmer (do not allow to boil). Pour the hot cream over the chocolate and let sit undisturbed for 2 minutes. Whisk gently until smooth and glossy. Allow to cool for 10 to 15 minutes, or until slightly thickened but still pourable.

Using a small paring knife or cupcake corer, remove a 1-inch-deep cavity from the center of each cupcake. Fill each cavity with the chilled pastry cream using a piping bag or spoon. Discard or reserve the cupcake centers.

Dip the top of each cupcake into the ganache, swirling gently to ensure even coating. Return cupcakes to the wire rack and allow the ganache to set at room temperature for 30 to 60 minutes before serving.