Boston Cream Pie Cupcakes

To make the pastry cream filling: In a small saucepan, bring 1 cup (227g) of the milk to a simmer over medium heat. 

While the milk is heating, whisk together the sugar, cornstarch, and salt in a medium bowl. Add the egg, yolk, and remaining 1/4 cup (57g) milk and whisk to combine.  

Whisk the egg and sugar mixture constantly while slowly streaming in half of the hot milk. This process tempers the eggs and prevents them from scrambling during the next step. 

Transfer the whisk to the pan containing the remaining hot milk and whisk constantly while pouring the tempered egg mixture back into the pan.  

Bring the mixture to a low boil over medium heat (this may happen very quickly) while whisking constantly. Once large bubbles begin to pop at the surface of the mixture, continue cooking (while still whisking) for 2 more minutes. The mixture will thicken significantly at this stage and begin to look more like pastry cream; the cornstarch will cook, eliminating grittiness in the filling. 

Remove the pastry cream from the heat and stir in the vanilla. 

Pass the pastry cream through a fine-mesh strainer and into a heatproof bowl to catch any small bits of egg. 

Place a piece of plastic wrap or parchment directly on the entire surface of the pastry cream so it doesn't form a skin. Let the bowl cool on the counter for about 20 minutes before placing it in the refrigerator to cool completely. (Pastry cream filling can be made and refrigerated up to 5 days ahead.)  

To make the cupcakes: Preheat the oven to 350°F with the rack in the center position. Line a standard muffin pan with baking cups.  

Using a stand mixer fitted with the flat beater or working with an electric hand mixer in a large bowl, mix the flour, sugar, baking powder, and salt on low speed until thoroughly combined. Add the butter and mix until evenly crumbly and the consistency resembles fine, damp sand, about 2 minutes. 

Beat in the eggs and egg white one at a time, beating for about 30 seconds on low speed and scraping down the sides and bottom of the bowl after each addition.  

In a small bowl or measuring cup, whisk together the milk and vanilla. Add the milk mixture, 1/3 at a time, to the batter. Beat for 30 seconds on low speed and scrape down the bowl after each addition. It’s OK if the batter looks a little curdled at this point. 

Portion the batter evenly into the wells of the prepared pan. Each well should contain about 1/4 cup (roughly 50g) of batter.  

Bake the cupcakes for 20 to 24 minutes, or until a toothpick or paring knife inserted into the center of a cupcake comes out clean or with a few moist crumbs sticking to it.  

Remove the cupcakes from the oven and let them cool in the pan for a few minutes, or until just cool enough to handle. Transfer to a wire rack. 

While the cupcakes are still slightly warm, use a paring knife or teaspoon scoop to cut a round hole about 1" in diameter and 1 1/2" deep in the center of each. (The scooped-out cake makes a great baker’s snack!) Set the cupcakes aside to cool completely while you make the frosting. 

To make the whipped ganache frosting: In a small saucepan, heat the cream over medium heat until it starts to simmer. Place the chocolate in a medium heat-proof bowl. 

Pour the hot cream over the chocolate and let sit undisturbed for 5 minutes. Then use a flexible spatula to stir — at first slowly, then more vigorously — until the chocolate is melted and the mixture is homogenous and shiny.  

Add the chocolate mixture to the bowl of a stand mixer fitted with the whisk. Starting on low speed, whisk the ganache to cool and aerate it. As the mixture thickens, increase the mixer speed to high and whip the ganache into a silky, pipeable consistency, about 4 to 6 minutes. Properly whipped frosting will be lighter in color, about twice the original volume, and slump slightly on the wires of the whisk if turned upright. (Be careful not to overwhip, as this will make it stiff and difficult to work with.) 

Add the frosting to a pastry bag fitted with a large open round tip (or no tip at all). If not using a tip, simply cut off the pointy end of the piping bag to create a round opening about 1 1/2" in diameter. Set aside. 

To fill the cupcakes: Place the cooled pastry cream filling into a bowl and whisk or stir vigorously by hand to break up any clumps and return the mixture to the silky, thick, pipeable consistency it was before it was refrigerated. 

Fill the center of each cupcake with about 1 heaping tablespoon (20g) of filling. You can use a tablespoon scoop or another pastry bag, fitted with a medium open round tip or snipped to create a 1" opening. 

To top the cupcakes: With the cupcakes on a flat surface, hold the piping bag containing the whipped ganache frosting in your dominant hand. Place its tip directly above (almost touching) the surface of one of the filled cupcakes at a 90° angle from the work surface.  

Squeeze the piping bag decisively and steadily to apply about 2 tablespoons (roughly 15g to 20g) of frosting onto the cupcake in a rounded dome that expands outward to cover almost the entire surface of the cupcake. Stop squeezing and quickly flick your wrist to create a small, decorative peak. Repeat with the remaining cupcakes.  

Refrigerate the cupcakes for 15 to 25 minutes, uncovered, to help the frosting set. 

To make the glaze: While the frosted cupcakes chill, heat the cream in a saucepan over medium heat until it starts to simmer. Place the chocolate in a medium heat-proof bowl. (The same saucepan and bowl used for the first batch of ganache can be used again without cleaning.) 

Pour the cream over the chocolate and let sit undisturbed for 5 minutes. Using a flexible spatula, stir — at first slowly, then more vigorously — until the chocolate is melted and the mixture is homogenous and shiny. 

Continue stirring until the glaze measures between 80°F to 85°F on a digital thermometer, or the consistency is relatively thick and falls from the spatula in ribbons that settle on the surface briefly before sinking back in. Tap the bottom of the bowl on the counter 5 to 10 times to pop any bubbles that may have formed during stirring. 

Remove the cupcakes from the refrigerator. Carefully dip the top of each cupcake into the glaze to cover the frosting. Lift the cupcake out of the glaze, rotating it to the side to allow any excess glaze to fall back into the bowl. Repeat the dipping process with the remaining cupcakes. The glaze will set up after a few minutes, creating a soft, shiny shell. Serve the Boston Cream Pie Cupcakes at room temperature.

Storage information: Store leftover filled and frosted Boston Cream Pie Cupcakes, covered, in the refrigerator for up to 2 days.