Step 1: In a large bowl, combine 1 cup (240ml) warm milk, ½ cup (113g) active sourdough starter, ¼ cup (50g) granulated sugar, and 2 large eggs. Whisk until well combined.
Step 2: Add 3 ½ cups (440g) all-purpose flour, ½ tsp salt, 4 tbsp (57g) softened unsalted butter, 1 tsp vanilla extract, and 1 tsp ground nutmeg (if using) to the wet ingredients.
Step 3: Mix with a spoon or your hands until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for 8-10 minutes until smooth and elastic. The dough will be soft.
Step 4: Place the dough in a lightly oiled bowl, cover tightly with plastic wrap or a damp towel, and refrigerate for 12-18 hours for a slow fermentation.
Step 5: The next day, remove the dough from the refrigerator. On a lightly floured surface, gently roll the dough out to about ½-inch thickness.
Step 6: Use a 3-inch donut cutter or two round cutters (one larger for the donut, one smaller for the hole) to cut out your donuts. Gather the scraps, gently re-knead them, let rest for 10 minutes, and cut out more donuts.
Step 7: Place the cut donuts and donut holes on parchment-lined baking sheets, leaving space between each. Loosely cover with plastic wrap or a clean kitchen towel.
Step 8: Let the donuts proof in a warm spot for 1-2 hours, or until visibly puffy and slightly larger. They should pass the 'poke test' – when gently poked, the indentation should spring back slowly.
Step 9: While the donuts are proofing, prepare your frying setup. Fill a heavy-bottomed pot or Dutch oven with 2-3 inches of vegetable oil. Heat the oil to 350-375°F (175-190°C) using a kitchen thermometer to monitor.
Step 10: Carefully slide 2-3 donuts into the hot oil (do not overcrowd the pot). Fry for 1-2 minutes per side, or until golden brown.
Step 11: Using a slotted spoon or spider, transfer the fried donuts to a wire rack set over a baking sheet to drain any excess oil. Repeat with the remaining donuts and holes, ensuring the oil temperature stays consistent.
Step 12: While the donuts are cooling slightly, prepare your glaze. In a small bowl, whisk together 1 ½ cups powdered sugar, 2-3 tbsp milk or cream, and ½ tsp vanilla extract until smooth. Add more milk if you prefer a thinner glaze.
Step 13: Dip the warm (but not hot) donuts into the glaze, letting any excess drip off, then return them to the wire rack. You can also toss them in cinnamon sugar or dust with powdered sugar.
Step 14: Let the glaze set for a few minutes before serving. Enjoy your incredible homemade sourdough donuts!