• Before you begin: Pull all the dairy and eggs for the yellow cake to room temperature. Preheat the oven to 350º F. Prep two 8-inch cake pans with cake goop or your preferred pan release. Have an 8-inch cake ring, an 8-inch cake board, and acetate ready for assembly.

Making the Pastry Cream

• In a medium bowl, whisk together the sugar, cornstarch, egg yolks, and whole egg until smooth.
• In a medium saucepan, combine the milk and vanilla. Bring to a simmer over medium heat.
• Temper the eggs by streaming about ⅓ of the hot milk into the egg mixture while whisking constantly.
• Pour the entire mixture back into the saucepan and cook over medium heat, whisking constantly, until thick and bubbling for a full minute.
• Strain the cream through a sieve into a large bowl while it is still warm (or use an immersion blender for the same result). Whisk in the butter until smooth.
• Press plastic wrap directly onto the surface of the cream and refrigerate until completely cold (at least 2 hours, overnight is better).

Making the Yellow Cake

• In a small bowl, combine half the milk (about 5 ounces), the vanilla, and the oil. Set aside.
• In a separate bowl, whisk together the eggs, yolks, and the remaining 3 ounces of milk. Set aside.
• In the bowl of a stand mixer with the paddle attachment, combine the cake flour, sugar, baking powder, baking soda, salt, and butter. Mix on low until the mixture resembles coarse sand.
• With the mixer on low, stream in the oil mixture. Bump the speed to medium (speed 4 on a KitchenAid) and mix for a full 2 minutes. Set a timer; this step builds the cake's structure.
• Stream in the egg mixture on low and mix until just homogeneous. Scrape the bowl.
• Divide the batter evenly between the two prepared cake pans (weighing the batter gets you even layers).
• Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
• Cool the cakes in the pans for 10 minutes, then flip them onto a wire rack. Flash freeze for an hour for same-day decorating, or wrap warm in plastic and freeze overnight.

Assembling the Boston Cream Pie

• Line an 8-inch cake ring with acetate (about 6 inches taller than the ring) on top of an 8-inch cake board.
• Trim the cake domes flat so the layers stack evenly.
• Place the first cake layer at the bottom of the ring.
• Pipe the chilled pastry cream over the first layer in an even ½-inch-thick disc.
• Set the second cake layer on top.
• Cover the cake loosely with plastic wrap and refrigerate at least 6 hours, preferably overnight.

Making Chocolate Ganache

• Place the chocolate in a heatproof bowl that pours easily. Microwave the cream in a separate bowl for about 1 minute, until steaming.
• Pour the hot cream over the chocolate and let it sit for 5 minutes.
• Whisk together until the ganache is smooth and shiny.
• Let the ganache cool to about 92 to 95º F (warm to the touch, not hot).
• Remove the metal ring and peel the acetate from the assembled cake.
• Pour the ganache over the top of the cake, nudging it toward the edges so it drips down the sides.
• Smooth the top with an offset spatula and refrigerate until the ganache sets.
• Slice with a clean, warm knife (run it under hot water and wipe between cuts).