Pastry Cream:

Heat milk in a saucepan over medium until it begins to steam and bubbles form around the edges. Remove milk from burner.

whisk sugar, cornstarch and salt together in a bowl. Add the eggs and whisk until smooth, then gradually whisk in 1/2 the hot milk.

Pour the egg milk mixture into the remaining milk in the pan and return pan to burner. Cook custard over medium heat, whisking often until thick. Boil 1 minute, whisking constantly to eliminate the starchy taste of the flour and to thicken further.

Turn off heat, stir in butter and vanilla until butter is melted. transfer to a bowl and cover with plastic, pressing it onto the surface to prevent a skin from forming. Chill before using.

Chocolate Glaze:

Boil cream and corn syrup in a saucepan. Off heat, whisk in chocolate until smooth.

Cupcakes:

Preheat oven to 325; line 16 muffin cups with paper liners and coat with non-stick spray.

Whisk dry flour, baking power and salt together in a bowl and set aside.

Cream sugar and butter in a bowl with a hand mixer on medium speed until fluffy, about 5 minutes. Add eggs 1 at a time, beating well after each addition. Blend in 1/2 the flour mixture, then add buttermilk and vanilla; mix until combined. Add remaining flour and mix until blended. Transfer 2 cups batter to a small bowl.

Fold cocoa and espresso mixture into cups batter until incorporated. Scoop a heaping tablespoon chocolate batter into the bottom of each liner and smooth to cover bottom. Top with vanilla batter, filling liners to the top, then bake until a toothpick inserted in center of a cupcake comes out clean, about 25 to 30 minutes.

Cool cupcakes in pan for 20 minutes tranfer to a rack, then when COMPLETELY cool, assembly cupcakes by cutting top off filling with scoop of pastry cream, placing top back on and top with 2 T of chocolate glaze.