This Boston Cream Pie is the best of the best. Each layer is delicious all on its own. The custard is rich, creamy, and smooth. The sponge cake layers are moist and tender. The ganache is amazing. It all comes together to form a ridiculous dessert you won't forget. Allow ample time for custard to chill and cake to cool before assembly.
Heat half and half in a saucepan over medium heat until simmering. Meanwhile, whisk yolks, sugar, and salt in a bowl until smooth. Add flour to yolk mixture and whisk until combined. Remove half and half from heat and - whisking constantly - very slowly add 1/2 cup of half and half to the yolk mixture, to temper. Whisking constantly, return tempered yolk mixture to the remaining half and half in the saucepan.
Return saucepan to medium heat and cook, whisking constantly, until it thickens, 1 min. Reduce to med-low and keep simmering, whisking constantly for 8 minutes. It will slowly go from pale yellow to a deep yellow and become thicker. Increase heat to medium and cook, whisking vidorously, until bubbles burst on surface, 1-2 min. Remove from heat and whisk in butter and vanilla until melted. It should be pudding-like, thick, and smooth (not clumpy.) Cover and chill at least 2 hours or overnight.
Sponge Cake
Preheat oven to 325F with rack on middle position. Grease two 9-inch round cake pans and line with parchment over the grease. In a bowl, whisk flour, baking powder, and salt. In a saucepan, heat milk and butter over low heat until butter melts. Remove from heat, add vanilla, and cover to keep warm.
In a stand mixer fitted with whisk attachment, whip the eggs and sugar at high speed until light and airy, 5 min. Add hot milk mixture and whisk by hand until combined. Add dry ingredients and whisk just until incorporated; don't overmix.
Quickly divide batter evenly between the two prepared pans. Bake until tops are golden brown and toothpick inserted in center comes out with small tender crumbs attached, about 20-27 min.
Transfer cakes to wire rack and cool completely in pan at room temp. Run thin sharp knife around edge of pans and invert cakes onto rack. You might need to give the pans a good slap on the back. Slowly and gently peel off the parchment paper.
Place one round cake on a serving plate, top side up. Stir chilled custard briefly and spread onto cake evenly. Place second round cake, bottom side up, on top of the custard. Cover and chill while you make the ganache.
Ganache
In a small saucepan over medium heat, bring heavy cream and corn syrup to simmer. Remove from heat and add chocolate chunks. Whisk until smooth. Let stand, whisking occasionally, until slightly thickened, 5-10 min. Whisk again and pour onto top of cake, using rubber spatula to spread evenly and letting it drip slowly down the sides. Cover and chill cake at least 3 hours before slicing. A fully chilled cake will be easier to slice.