There's nothing I cherish more than traditional European culinary delights, especially sourdough doughnuts!
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.
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.
Let the dough rise at room temperature for at least 10 hours ideally overnight for 12 hours. This slow rise adds flavor and texture.
Chill the risen dough in the fridge for about 2 hours to firm up, making it easier to handle and shape.
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.