STEP 1: In the bowl of a stand mixer, begin by mixing your sourdough starter, milk, egg yolks, sugar, salt, and both flours in a stand mixer with a dough hook. Start on low to get everything combined nicely.

STEP 2: Let the dough rest covered for about 5-10 minutes, so the flour has a chance to absorb the liquids. Then knead it on low, adding half the butter until it's absorbed, then add the remaining butter. Mix for about 15 - 20 minutes overall. Aim for a smooth, shiny dough that passes the windowpane test.

STEP 3: Let the dough rise at room temperature for at least 10 hours ideally overnight for 12 hours. This slow rise adds flavor and texture.

STEP 4: Chill the risen dough in the fridge for about 2 hours to firm up, making it easier to handle and shape.

STEP 5: On a lightly floured surface, divide the chilled dough into 10 pieces, each about 55g. Roll them into smooth balls and place them in a dish lined with a floured tea towel or floured parchment paper. Cover them with plastic wrap, so they won't dry out. Use a dish that is deep enough. That way, when the dough balls expand during the bulk rise, you won't have to worry about them sticking to the plastic wrap.

STEP 6: Let the dough balls proof for another 8-10 hours in a warm spot until they've nearly tripled in size, getting ready for their big fry.

STEP 7: Heat your frying oil (vegetable oil is a good choice) to 360°F. Fry the donuts in batches, turning once, for 2 minutes on each side.

STEP 8: Drain the fried donuts on a rack with paper towels under them.

STEP 9: When cool enough to handle but still warm, sprinkle them with powdered sugar or roll them in granulated (regular) sugar. They're ready to enjoy, best served the same day for that perfect soft bite!