In a small bowl, mix warm milk, yeast, and 1 teaspoon of sugar. Let sit for 5–10 minutes until foamy.

In the bowl of a stand mixer fitted with the dough hook, whisk together flour and salt.

Add ¼ cup sugar, eggs, and the activated yeast mixture. Knead on low speed for 4 minutes until it starts to come together.

Add softened butter, one tablespoon at a time, letting each piece fully incorporate before adding the next. This should take 2–3 minutes.

Knead on medium speed for 6–8 more minutes until smooth, stretchy, and slightly sticky.

If it still feels too sticky, let the dough rest 5 minutes, then knead for 1 more minute to allow the flour to absorb the butter.

Transfer to a greased bowl, cover, and refrigerate overnight (6–12 hours).

In a medium saucepan, heat milk, sugar, and vanilla over medium-high until bubbling and foamy, 3–5 minutes. Watch closely so it doesn’t boil over.

In a medium bowl, whisk together egg yolks and cornstarch until smooth.

Remove the milk mixture from the heat and add the espresso powder (doing it here reduces the chance of burning the espresso).

Slowly drizzle about ½ cup of the hot milk mixture into the egg mixture, whisking constantly to temper the eggs. The mixture should stay smooth and slightly warm, not scrambled.

Return the saucepan with the remaining milk to the stove. Add the tempered egg mixture while whisking quickly.

Reduce heat to medium and cook, whisking continuously, until the custard thickens. It will look like thick pudding but a little stiffer — you’ll feel more resistance as you stir, almost like smooth brownie batter. You may see slow, thick bubbles plop up in the center when it’s ready.

Remove from heat, transfer to a heatproof bowl, and press plastic wrap directly onto the surface to prevent a skin from forming.

Cool 20–30 minutes at room temperature, then refrigerate overnight until fully chilled. You’ll finish the rest when assembling the donuts.

On a floured surface, roll the chilled dough to ½-inch thick.

Cut out 9 rounds with a 3-inch cutter. Twist scraps into crullers instead of re-rolling.

Place on a lined tray, cover loosely, and let rise 1–2 hours until puffy and room temperature.

Heat oil in a deep pot to 325°F.

Fry donuts in batches, 60–90 seconds per side, until golden.

Transfer to a paper towel-lined wire rack to cool completely before filling.

In a stand mixer fitted with the whisk attachment, whip ⅔ cup of the heavy cream to medium peaks (the peaks should hold their shape but curl slightly at the tip).

Add the chilled custard and mix until smooth.

Gently fold in the remaining ⅓ cup heavy cream with a spatula until evenly combined.

Transfer to a piping bag.

Use a chopstick or wooden spoon handle to poke a hole in the top (or side) of each donut and wiggle to create a pocket.

Place the powdered sugar in a shallow dish and gently roll all sides of each donut in it.

Pipe custard into each donut until it puffs slightly.

Dust with espresso powder, cocoa powder, or more powdered sugar before serving.