In a large mixing bowl cream together the butter and sugar. Add vanilla and beat again. Add eggs, one at a time, beating well after each addition. Mix together flour, baking powder and salt in a separate bowl. Alternate adding flour mixture and buttermilk to the creamed mixture in 3 batches, beginning and ending with the dry ingredients. Beat after each addition, just until combined.

Line a lightly greased muffin pan with cupcake liners and fill each liner ¾ of the way full. Bake in a 350°F oven for 18-25 minutes or until a toothpick comes out clean. Allow to cool completely.

In a medium bowl, beat egg yolks well. Gradually stir in milk until well blended. In a large, heavy saucepan over medium-low heat, mix together sugar, cornstarch, and salt. Gradually add in milk mixture, stirring well to prevent lumps. Continue cooking over medium-low heat, stirring constantly until mixture reaches a low boil (about 20 minutes). Remove from heat and set pot in a large bowl of ice water, stirring. Add vanilla and stir to combine. Refrigerate until ready to use.

Place chocolate in a food processor and process until finely chopped. Heat cream in a small saucepan until steaming, then add hot cream to the bowl of chocolate and whisk until smooth. Let cool until handleable.

When the cupcakes are cooled completely, transfer pastry cream filling to a piping bag with filling tip attached. Fill each cupcake with pastry cream.

Dip each cupcake into the ganache. Allow 30 minutes to an hour for the ganache to set.