A hot-milk sponge cake made a good base in our Boston Cream Pie recipe because it didn’t require any finicky folding or separating of eggs. Baking the batter in two pans eliminated the need to slice a single cake horizontally before adding the filling. We used butter to firm up our pastry cream, and we added corn syrup to heavy cream and melted chocolate for a smooth glaze that clung to the top of our Boston Cream Pie and dripped artistically down its sides.
Chill the assembled cake for at least 3 hours to make it easy to cut and serve.
1. For the Pastry Cream: Heat half-and-half in medium saucepan over medium heat until just simmering. Meanwhile, whisk yolks, sugar, and salt in medium bowl until smooth. Add flour to yolk mixture and whisk until incorporated. Remove half-and-half from heat and, whisking constantly, slowly add 1/2 cup to yolk mixture to temper. Whisking constantly, return tempered yolk mixture to half-and-half in saucepan.
2. Return saucepan to medium heat and cook, whisking constantly, until mixture thickens slightly, about 1 minute. Reduce heat to medium-low and continue to simmer, whisking constantly, 8 minutes.
3. Increase heat to medium and cook, whisking vigorously, until bubbles burst on surface, 1 to 2 minutes. Remove saucepan from heat; whisk in butter and vanilla until butter is melted and incorporated. Strain pastry cream through fine-mesh strainer set over medium bowl. Press lightly greased parchment paper directly on surface and refrigerate until set, at least 2 hours and up to 24 hours.
4. For the Cake: Adjust oven rack to middle position and heat oven to 325 degrees. Lightly grease two 9-inch round cake pans with nonstick cooking spray and line with parchment. Whisk flour, baking powder, and salt together in medium bowl. Heat milk and butter in small saucepan over low heat until butter is melted. Remove from heat, add vanilla, and cover to keep warm.
5. In stand mixer fitted with whisk attachment, whip eggs and sugar at high speed until light and airy, about 5 minutes. Remove mixer bowl from stand. Add hot milk mixture and whisk by hand until incorporated. Add dry ingredients and whisk until incorporated.
6. Working quickly, divide batter evenly between prepared pans. Bake until tops are light brown and toothpick inserted in center of cakes comes out clean, 20 to 22 minutes.
7. Transfer cakes to wire rack and cool completely in pan, about 2 hours. Run small plastic knife around edge of pans, then invert cakes onto wire rack. Carefully remove parchment, then reinvert cakes.
8. To Assemble: Place one cake round on large plate. Whisk pastry cream briefly, then spoon onto center of cake. Using offset spatula, spread evenly to cake edge. Place second layer on pastry cream, bottom side up, making sure layers line up properly. Press lightly on top of cake to level. Refrigerate cake while preparing glaze.
9. For the Glaze: Bring cream and corn syrup to simmer in small saucepan over medium heat. Remove from heat and add chocolate. Whisk gently until smooth, 30 seconds. Let stand, whisking occasionally, until thickened slightly, about 5 minutes.
10. Pour glaze onto center of cake. Use offset spatula to spread glaze to edge of cake, letting excess drip decoratively down sides. Chill finished cake 3 hours before slicing. Cake may be made up to 24 hours before serving.