Preheat the oven to 350°F (175°C). Lightly grease a 12-cup muffin tin and line with cupcake liners.

In a large mixing bowl, cream the softened butter and granulated sugar on medium speed until light and fluffy (3–4 minutes).

Add the vanilla extract and beat to combine. Add the eggs one at a time, beating thoroughly after each addition until fully incorporated.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the butter mixture in three parts, alternating with the buttermilk in two parts. Begin and end with the dry ingredients, mixing on low speed until just combined. Do not overmix.

Divide the batter evenly among the prepared liners, filling each ¾ full. Bake for 18–25 minutes, or until a toothpick inserted into the center comes out clean. Transfer to a wire rack to cool completely.

In a medium bowl, vigorously whisk the egg yolks until smooth. Gradually whisk in the milk until fully blended. Set aside.

In a heavy-bottomed saucepan, combine the sugar, cornstarch, and salt. Gradually pour in the milk mixture while whisking constantly to prevent lumps.

Cook over medium-low heat, stirring continuously with a wooden spoon or silicone spatula, until the mixture thickens and reaches a low boil (about 15–20 minutes).

Immediately remove from heat and stir in the vanilla extract. Transfer the pastry cream to a heatproof bowl and press a sheet of plastic wrap directly onto the surface to prevent a skin from forming. Chill in the refrigerator for at least 1 hour.

Place the chopped chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it begins to simmer (do not boil).

Pour the hot cream over the chocolate and let sit undisturbed for 2 minutes. Whisk gently until smooth and glossy. Allow the ganache to cool slightly, stirring occasionally, until thickened but still pourable (about 10–15 minutes).

Once the cupcakes are fully cooled, use a small paring knife or cupcake corer to remove a 1-inch portion from the center of each. Discard or reserve the scraps.

Transfer the chilled pastry cream to a piping bag fitted with a round tip. Fill each cupcake cavity with pastry cream until just level with the surface.

Dip the top of each cupcake into the ganache, gently swirling to coat evenly. Place on a wire rack and allow the ganache to set at room temperature for 30–60 minutes before serving.