For the Pastry Cream:
1⅓ cups heavy cream
3 egg yolks
⅓ cup granulated sugar
⅛ tsp salt
4 teaspoons cornstarch
2 tablespoons unsalted butter, chilled and cut into 2 pieces
1½ teaspoons vanilla extract
For the Cupcakes:
1¾ cups all-purpose flour
1 cup granulated sugar
1½ teaspoons baking powder
¾ teaspoon salt
12 tablespoons unsalted butter, softened but still cool, cut into 12 pieces
¾ cup whole milk
1½ teaspoons vanilla extract
For the Chocolate Glaze:
¾ cup heavy cream
¼ cup light corn syrup
8 ounces bittersweet chocolate, finely chopped
½ teaspoon vanilla extract
Make the Pastry Cream: Heat the heavy cream in a medium saucepan over medium heat until simmering, stirring occasionally. Meanwhile, whisk the egg yolks, sugar and salt together in a medium bowl. Add the cornstarch and whisk until the mixture is pale yellow and thick, about 15 seconds.
When the cream reaches a full simmer, slowly whisk it into the yolk mixture. Return the mixture to the saucepan and cook over medium heat, whisking constantly, until it's a thick pudding consistency, 1 to 2 minutes. Remove the pan from the heat and whisk in the butter and vanilla extract until the butter is completely melted. Transfer the pastry cream to a small bowl and press plastic wrap directly on the surface. Refrigerate until set, at least 2 hours or up to 2 days.
Make the Cupcakes: Preheat oven to 350 degrees F. Line a standard 12-cup muffin pan with paper liners and set aside. In the bowl of a stand mixer fitted with paddle attachment, combine the flour, sugar, baking powder and salt on medium-low speed. Add the butter and mix until the mixture resembles coarse sand. Add the eggs, gently mixing until incorporated. Add the milk and vanilla extract; to avoid splatter, blend on low and then increase to medium, mixing until light and fluffy and no lumps remain, about 3 minutes, stopping to scrape the sides and bottom of the bowl halfway through. Batter will be almost a bit "foamy" to the touch.
Fill the muffin cups three-quarters full and bake until a toothpick inserted into the center of a cupcake comes out clean, 19-20 minutes. Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
Make the Glaze: Combine the heavy cream, corn syrup, chocolate and vanilla in a small saucepan over medium heat, stirring constantly, until smooth. Set the glaze aside to cool and thicken at room temperature at least 1 hour (I let mine sit longer to thicken to a frosting consistency. Chill in fridge a bit, if needed.)
Assemble the Cupcakes: Using a paring knife, cut into the center of the cupcake at a 45-degree angle about ⅛-inch from the edge and cut all the way around. Remove the cone and cut away all but the top ¼ inch, leaving a small disk of cake.
Fill the inside of each cupcake with room-temp pastry cream and top with the disk of cake. Spread the chocolate glaze over top of each cupcake, covering the top completely. Refrigerate the cupcakes until the glaze is set, about 10 minutes. Serve at room temp.