Sourdough Boston Cream Donuts

Prepare the custard ahead of time so it has plenty of time to chill before filling the donuts.
In a medium bowl, whisk together the egg yolks and cornstarch until completely smooth. Set aside.
In a medium saucepan over medium heat, combine the milk, sugar, and salt. Whisk frequently and heat until the mixture is steaming but not boiling.
Scoop out about 1 cup (240 g) of the hot milk mixture and slowly drizzle it into the egg mixture while whisking constantly. This tempers the eggs and prevents scrambling.
Gradually pour the tempered egg mixture back into the saucepan with the remaining milk, whisking continuously.
Cook over medium heat, whisking constantly, for 3–5 minutes or until the custard becomes thick and smooth.
Remove from the heat and stir in the vanilla extract.
Transfer the custard to a clean bowl and press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate for at least 2 hours or until completely chilled.

Add the sourdough starter, warm milk, flour, eggs, salt, sugar, and vanilla to the bowl of a stand mixer fitted with the dough hook attachment.
Mix on low speed until the ingredients come together into a shaggy dough.
With the mixer running on low, add the softened butter one tablespoon at a time, allowing each piece to fully incorporate before adding the next.
Increase the speed to medium-high or high and knead for about 7 minutes, or until the dough is smooth, elastic, and pulling away from the sides of the bowl.
The dough will be very soft and slightly sticky. Avoid adding extra flour, as this helps create light and fluffy donuts.

Transfer the dough to a large lightly greased bowl and cover tightly with plastic wrap.
Let the dough bulk ferment for up to 6-10 hours, depending on the temperature of your kitchen.
The dough should rise about 50–60%. Brioche doughs naturally ferment more slowly due to the butter and eggs. Once it bulk ferments, you can continue on to shaping and giving them a second rise, or you can cover tightly and refrigerate overnight to make the next day.

Turn the dough out onto a lightly floured surface.
Roll the dough to about 1/2-inch thickness. Using a round biscuit cutter or the rim of a glass, cut out 12 donuts. Depending on the size of your cutter, you may be able to get a couple extra donuts.
Place each donut onto an individual square of parchment paper and transfer them to a baking sheet.

Cover the donuts loosely and let them rise in a warm place for 2–3 hours, or until noticeably puffy.
To test if the donuts are ready, gently press your finger into the dough. If the indentation slowly springs back, the donuts are fully proofed. If it springs back quickly, they need more time.

Add coconut oil or another neutral oil to a heavy-bottomed pot, filling it 2–3 inches deep.
Heat the oil to 340–360°F. Maintaining the proper oil temperature is important to ensure the donuts cook evenly without burning.
Carefully lower the donuts into the hot oil using the parchment squares. The parchment will naturally release after a few seconds and can be removed with tongs.
Fry the donuts in batches for 1–2 minutes per side, or until deeply golden brown.
Transfer the fried donuts to a wire rack lined with paper towels underneath to catch excess oil. Allow the donuts to cool completely before filling.

In a small saucepan over low heat, warm the heavy cream until steaming but not boiling.
Add the chocolate chips and butter, whisking continuously until completely melted and smooth.
Remove from the heat and stir in the vanilla extract. Let the ganache cool slightly until thick, glossy, and perfect for dipping.

Transfer the chilled custard to a piping bag fitted with a round tip.
Using a small knife or skewer, poke a hole into the side of each donut. Pipe the vanilla custard into the center until the donut feels full and slightly heavy.
Dip the tops of the filled donuts into the glossy chocolate ganache, allowing any excess to drip off.
Let the ganache set for a few minutes before serving.