Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans.
In a bowl, combine flour, baking powder, and salt. Set aside.
In another bowl, cream butter and granulated sugar (cake). Add eggs one at a time, then stir in vanilla.
Alternate adding flour mixture and buttermilk to the butter mixture, beginning and ending with flour. Pour batter into prepared pans.
Bake 25-30 minutes or until a toothpick inserted in centers comes out clean. Cool completely on wire racks.
For custard, whisk 1/2 cup sugar, yolks, egg, and cornstarch in a bowl. In a saucepan, gently heat milk and remaining 1/6 cup sugar until warm. Slowly add milk to egg mixture, whisking constantly.
Return mixture to saucepan. Cook over medium heat 8-10 minutes, stirring constantly, until thickened. Remove from heat, stir in 4 tbsp butter until smooth. Chill 30 minutes.
For chocolate ganache, heat heavy cream, sugar, and 2 tbsp butter in a heatproof bowl over simmering water. Add chocolate and stir until smooth. Whisk until glossy.
Place one cake layer on a plate. Spread custard evenly over. Add second layer. Drizzle ganache over top and sides. Refrigerate 30 minutes before serving.