Semi-Homemade Boston Cream Pie

Preheat the oven to 350 degrees F (175 degrees C). Spray two 9-inch round metal or glass cake pans with cooking spray.

Combine cake mix, water, eggs, and vegetable oil in a large bowl; beat with an electric mixer on low speed until moistened. Increase speed to medium and beat for 2 minutes. Pour batter into the prepared cake pans.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 23 to 28 minutes. Cool on a wire rack for 5 minutes. Run a table knife around the edges to loosen. Invert carefully onto a serving plate or cooling rack. Let cool completely, then shave the top of each layer with a serrated knife or dental floss to flatten.

While the cakes are cooling, combine sugar, cornstarch, and salt in a pot. Whisk in milk and egg yolks and cook over medium heat until boiling, whisking continuously to prevent lumps. Boil and stir until thickened, 3 to 5 minutes. Remove from the heat, stir in vanilla extract, and set custard aside to cool completely.

While custard is cooling, bring whipping cream to a boil in a large pot over medium heat. Turn off the heat and add chocolate; stir until melted.

Place first cake layer on a serving plate, spread all of the custard evenly over top, and cover with second cake layer. Slowly pour panache over the top of the cake, allowing chocolate to drizzle down the sides. Place in the refrigerator until ready to serve.