For the Boston Cream Filling:
In a medium-sized bowl, whisk together the egg yolks, sugar, and cornstarch. Set aide.
In a medium-sized saucepan, warm the milk over medium-heat until it comes to a gentle boil.
Remove the pan from heat and slowly whisk 1 cup of the hot milk into the egg yolk mixture, whisking constantly.
Return the mixture to the pan containing the remaining milk.
Place over medium-high heat and cook, whisking constantly, until it comes to a boil. Once boiling, continue to cook, whisking constantly, for 2 minutes, or until very thick.
Remove from heat. Scrape the bottom and sides of the pan with a spatula, then whisk smooth.
Whisk in the butter, whisking until completely melted. Then whisk in the vanilla and salt.
Strain the custard through a fine-mesh sieve and into a medium-sized glass bowl.
Cover with plastic wrap, pressing the wrap directly onto the surface to avoid a skin forming.
Cool to room temperature, then place in the refrigerator for at least 2 hours, or until well chilled.
Boston cream filling will keep, stored in the fridge, for up to 4 days!
When ready to fill donuts, scrape the filling into a large pastry bag fitted with a small round nozzle.
Using a large straw, or your pinky finger, make a small hole on the side of each doughnut.
Carefully fit the nozzle of the bag into a doughnut and squeeze until you see the center begin to puff up. You can add a lot of filling, just be sure not to add so much the donut tears. Repeat with all donuts.
For the Chocolate Glaze:
Place the finely chopped chocolate (or chocolate chips) in a medium-sized heatproof bowl. Set aside.
Warm the cream over medium heat until it comes to a boil. Add in the espresso powder, if using, and whisk smooth.
Pour the cream over the chopped chocolate and let it sit for 1 minute, then whisk smooth.
Dip the top of each donut into the glaze, allowing excess chocolate to drip off before placing it on a large serving platter. Repeat with all donuts. Allow glaze to set, about 20 minutes. You can speed this up by placing the donuts in the fridge.
Donuts are best served the day they are made. Store leftovers in the fridge, wrapped in saran wrap, for up to 48 hours.
NOTES
Donut dough can be made the day before you plan on using it. Instead of placing in a warm area to rise, cover the bowl and place in the fridge for at least 2 hours, and up to 24 hours. Then follow the recipe as directed.
