Preheat your oven to 350°F (175°C). In a medium bowl, whisk together the flour, baking powder, and salt. In a large bowl, beat sugar, eggs, milk, and vanilla until smooth. Gradually mix in dry ingredients until just combined.

Pour the batter evenly into two greased 9-inch round cake pans. Bake for 25-30 minutes, until tops are golden and a toothpick comes out clean. Cool in pans for 10 minutes before transferring to wire rack.

For the custard, heat milk until steaming. Whisk together egg yolks and sugar until pale, then gradually add hot milk while whisking. Return to saucepan and cook, stirring continuously, until thickened. Cool.

For the ganache, heat heavy cream just before boiling. Remove from heat, pour over chopped chocolate, and stir until smooth. Let it cool slightly.

Assemble the Boston Cream Pie by placing one cake layer on a serving plate. Spread half of the custard over it, then add the second layer. Top with remaining custard, letting it drip down the sides.

Pour chocolate ganache over the pie to cover evenly. Refrigerate for at least 1 hour before serving.