Preheat the oven to 350°F (175°C). Grease and line two 8-inch cake pans and set aside.
Sift together the Cake Flour, Baking Powder, and Fine Salt into a medium bowl and set aside.
In a large bowl, cream the Unsalted Butter and Granulated Sugar until pale and fluffy, then beat in the Eggs one at a time, followed by the Vanilla Extract.
With the mixer on low, add the dry ingredients in three additions alternately with the Whole Milk, beginning and ending with the dry mix, mixing just until combined.
Divide the batter evenly between the prepared pans and bake until a toothpick comes out clean and the tops are golden, about 22–30 minutes; let the cakes cool in the pans 10 minutes, then turn out onto a rack to cool completely.
While the cakes bake, make the pastry cream: heat most of the Whole Milk with half of the Granulated Sugar in a saucepan until it just begins to simmer.
Whisk the Egg Yolks with the remaining Granulated Sugar, the Cornstarch, and a pinch of Fine Salt until smooth and pale.
Temper the yolk mixture by slowly whisking in a ladleful of the hot milk, then return the tempered mixture to the saucepan with the remaining milk and cook over medium heat, whisking constantly, until the mixture thickens and comes to a boil.
Remove from the heat and stir in the Unsalted Butter and Vanilla Extract, then transfer the pastry cream to a bowl, press plastic directly onto the surface to prevent a skin, and chill until set.
Make the ganache by chopping the Semisweet Chocolate. Heat the Heavy Cream with the Light Corn Syrup until hot and just simmering, pour over the chocolate, let sit for 1–2 minutes, then stir until smooth and glossy; stir in a little Unsalted Butter for extra shine if desired.
To assemble, level the cooled cakes if needed, place one layer on your serving plate, spread a thick, even layer of the chilled pastry cream over it, top with the second cake layer, and pour the warm ganache over the top, allowing it to drip down the sides.
Chill the assembled Boston Cream Pie for at least 1 hour to set the filling and ganache before slicing and serving.
