Cream the softened butter and sugar in a large bowl until light and fluffy. Add vanilla and beat again to incorporate. Introduce the eggs one by one, ensuring each is fully blended before adding the next.

In a separate mixing bowl, combine flour, baking powder, and salt. Gradually add the dry ingredients to the creamed mixture in three additions, alternating with the buttermilk, beginning and ending with the dry mix. Beat gently after each addition, just until combined.

Line a muffin tin with paper cupcake liners and lightly grease them. Fill each cavity about three-quarters full. Bake at 350°F for 18 to 25 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool entirely.

In a medium bowl, whisk the egg yolks thoroughly. Gradually blend in the milk until smooth. In a heavy-bottomed saucepan, mix sugar, cornstarch, and salt. Slowly add the milk mixture while stirring continuously to avoid lumps.

Cook over medium-low heat, stirring constantly until the mixture reaches a low boil and thickens, approximately 20 minutes. Remove from heat and place the pan in an ice water bath, stirring to help it cool. Stir in vanilla, then press a piece of plastic wrap directly onto the surface and refrigerate until ready to use.

Place chopped chocolate in a bowl. Heat the cream in a small saucepan until steaming, then pour over the chocolate. Let sit briefly, then whisk until the ganache is glossy and smooth. Allow to cool until thickened but still spreadable.

Once cupcakes are fully cooled, pipe the chilled pastry cream into the center of each using a piping bag fitted with a filling tip. Dip the tops into the ganache and let them sit for 30 to 60 minutes to set.