While it has the word pie in its name, Boston cream pie is simply a yellow cake with pastry cream in the middle and a chocolate ganache spread on top. The richness of the chocolate ganache is tempered when combined with the moist, tender yellow cake and the lightly sweet pastry cream, creating an all-around satisfying dessert. Whether it's made for a special occasion or any day of the week, you'll love this cake.
Preheat the oven to 350°F. Butter two 9-inch round cake pans, fit with parchment in the bottom, and set aside.
In the bowl of a stand mixer, beat the butter and sugar until light and fluffy, 3-5 minutes. Scrape the sides of the bowl. With the mixer running on low, add the egg yolks and eggs one at a time until each one is mixed in. Scrape the sides and bottom of the bowl.
In a separate bowl, combine the flour, baking powder, and salt. Whisk to combine.
In a small measuring glass, whisk the buttermilk, vanilla, and oil.
With the mixer running on low, add ⅓ of the dry mixture followed by ½ of the milk mixture. Allow to incorporate and repeat the steps, finishing with the final ⅓ of the dry mixture.
Divide the cake batter between the two cake pans and smooth out to an even layer. Bake in the preheated oven until a skewer inserted in the middle comes out clean, 24-26 minutes.
Once baked, remove the cakes from the oven. Allow the cakes low to cool in the pans for 5 minutes. Once slightly cooled, remove the cakes from the pans, discard the parchment paper, and cool completely on a wire rack.
For the pastry cream
In a 4-quart saucepan, combine the egg yolks, milk, sugar, salt, and cornstarch. Whisk until the mixture is smooth. Set the mixture over medium-low heat. Whisk constantly until the mixture thickens. Continue to whisk until mixture is thick with bubbles forming, 6-8 minutes.
Once thickened, remove the pastry cream from the heat. Add the butter and vanilla. Whisk until the butter dissolves and immediately push through a sieve. Allow the pastry cream to cool. I find this best to make the day before and store in the refrigerator.
For the ganache
Chop the chocolate into fine, uniform pieces and pour them into a small bowl. Heat the heavy cream until it's steaming and almost boiling. Once the cream is steaming, immediately pour the cream over the prepared chocolate. Allow the chocolate and cream to rest for 5 minutes.
Once the mixture has rested, slowly stir until smooth. Stir in the vanilla. Allow the mixture to cool to room temperature. As the ganache cools, it will thicken.
To assemble the cake
Place a room temperature or cool layer of cake on a serving platter. Spread the chilled pastry cream over one layer of the cake, staying about ½-inch from the edge. Place the second layer on top of the pastry cream. Pour the ganache over the top layer. If the ganache is too thick, heat for a few seconds in the microwave. Allow the ganache to slightly drip down the sides. Serve immediately.