Boston Cream Donuts

For the filling:

• 1/2 cup heavy cream
• 1/2 cup whole milk
• 1/4 cup granulated sugar
• Pinch of fine sea salt
• 3 large egg yolks
• 1 1/2 tablespoons cornstarch, mixed with 2 tablespoons water
• 1 teaspoon pure vanilla extract
• 2 tablespoons (1/4 stick) cold unsalted butter, cut into pieces

For the donuts:

• 1 recipe Basic Yeast Donuts

For the glaze:

• 2 cups powdered sugar, also known as confection
• ers’ sugar, sifted
• 1/3 cup natural unsweetened cocoa powder, sifted
• Pinch of fine sea salt
• 2 tablespoons whole milk
• 2 teaspoons pure vanilla extract

To make the filling:
1In a saucepan, stir together the cream, milk, 2 tablespoons of the sugar, and the salt. Warm over medium-low heat until gently simmering, stirring to dissolve the sugar.
2In a heatproof bowl, whisk together the egg yolks and the remaining 2 tablespoons of sugar. Whisk in the cornstarch mixture. Slowly pour about half of the warm cream mixture into the yolk mixture, whisking constantly.
3Scrape the yolk mixture into the saucepan. Cook, stirring constantly, until the custard is thickened and glossy, about 3 minutes.
4Remove from the heat and stir in the vanilla extract and the butter until melted. Pour the pastry cream through a medium-mesh sieve into a bowl, press a piece of plastic wrap directly onto the surface of the custard, and let cool completely. (You can cool the custard down quickly by setting the bowl in a larger bowl filled with ice water.)
5Refrigerate until chilled before using; the pastry cream can be kept in the refrigerator, covered, for up to 3 days.
To make and fill the donuts:

1Mix the dough, let it rise, and roll it out to a 1/4-inch thickness following the directions in step 1. Cut out rounds using a 2-1/2- to 3-inch biscuit cutter with smooth sides. Follow the directions to proof and fry the donuts.
2To make the glaze, whisk together the powdered sugar, cocoa powder, and salt. Slowly add the milk and vanilla and whisk until the glaze is very smooth. Cover with plastic wrap until ready to use. (Use within 1 hour.)
3Working with 1 donut round at a time and using a paring knife or a wooden skewer, carefully carve a small opening in the side of each round, forming a cavity. Fill a piping bag fitted with a small tip or a plastic bag with a small corner cut out with the chilled pastry cream. Pipe a few tablespoons into each donut round.
To finish:

1Glaze the filled donuts, and then transfer to a rack to let the glaze set.