In a medium pot, combine the peeled, diced apples, lemon juice, sugar, cinnamon, and salt. Stir with a rubber spatula to coat the apples evenly before turning on the heat.

Place pot over medium-high heat and cook, stirring often, until the apples release liquid and start to soften (5–7 minutes). They should look glossy and slightly tender.

In a small bowl, whisk cornstarch and cold water until smooth (this is your slurry).

Stir slurry into the apple mixture. Keep stirring constantly—within 30 seconds it should thicken into a glossy pie filling. If not, make a bit more slurry and add it.

Remove from heat. Transfer to a heatproof container and press plastic wrap directly on the surface. Chill in the fridge until completely cooled (up to 1 week).

In a small bowl, mix the warm milk, yeast, and 1 teaspoon sugar. Let sit 5–10 minutes until foamy on top (this means the yeast is alive).

In a stand mixer fitted with a dough hook, combine flour, salt, remaining sugar, eggs, and yolks. Pour in the foamy yeast mixture. Mix on low speed for 4 minutes until dough starts to form.

Add softened butter, 1 tablespoon at a time, letting it incorporate before adding the next. This should take 2–3 minutes.

Increase speed to medium and knead 6–8 minutes, until dough is smooth, stretchy, and slightly tacky (it should pull away from the sides of the bowl but still cling a bit at the bottom).

Transfer dough to a lightly greased bowl. Cover tightly and refrigerate overnight (6–12 hours) for flavor and structure.

In a medium saucepan, heat milk, sugar, and vanilla over medium heat until steaming and small bubbles appear around the edges (do not boil).

In a separate bowl, whisk yolks and cornstarch until smooth and pale. Slowly drizzle in ½ cup of the hot milk, whisking constantly, to temper the eggs.

Pour tempered mixture back into saucepan while whisking. Cook over medium heat, whisking nonstop, until thick and pudding-like (3–5 minutes). You’ll know it’s ready when thick bubbles pop slowly in the center and it coats the back of a spoon.

Transfer to a heatproof bowl, press plastic wrap directly onto the surface, and chill overnight until fully cold.

The next day, lightly flour your counter. Roll chilled dough into a ½-inch thick rectangle.

Use a 3-inch round cutter to cut 9 donuts. Gather scraps and twist into crullers instead of re-rolling.

Place on a parchment-lined baking sheet. Cover loosely with plastic wrap and let rise 1–2 hours, until puffy and airy.

In a heavy pot, heat oil to 325°F. Use a thermometer—too cool = greasy donuts, too hot = burnt outside/raw inside.

Fry 2–3 donuts at a time, 60–90 seconds per side, until evenly golden brown.

Use a slotted spoon to transfer to a paper towel-lined wire rack. Let cool completely before filling.

In a large bowl, whisk powdered sugar, salt, vanilla, melted butter, cinnamon, milk, and corn syrup until smooth and pourable. Add more milk 1 teaspoon at a time if too thick.

In a stand mixer, whip ⅔ cup heavy cream to medium peaks (they should curl slightly at the tips).

Add chilled custard and beat until smooth. Gently fold in apple filling, then fold in the remaining ⅓ cup cream until light and fluffy.

Transfer filling to a piping bag fitted with a round tip.

Use a chopstick or the handle of a wooden spoon to poke a hole in each donut, wiggling to make a pocket.

Pipe about 5 tablespoons of filling into each donut until it feels heavy and slightly puffy.

Spoon glaze over each donut, letting it drip down the sides. Set for 10 minutes before serving.