For the cake:
Cooking spray
2 cups unbleached all-purpose flour
2 cups granulated sugar
1/2 cup unsweetened cocoa powder (see Recipe Note)
1 1/2 teaspoons baking soda
3/4 teaspoon fine salt
1 cup well-shaken buttermilk, at room temperature
1/2 cup hot brewed coffee or hot water
1/2 cup vegetable or canola oil
2 large eggs
2 teaspoons vanilla extract
For the filling:
1 cup granulated sugar
1/4 cup cornstarch
1/8 teaspoon fine salt
1 cup whole milk
1 teaspoon vanilla extract
8 tablespoons (1/2 cup) unsalted butter, at cool room temperature