To make the dough, combine your sugar, milk, salt, eggs, butter, and the bubbly sourdough starter in a mixing bowl of a stand mixer. Use the dough hook attachment and mix on low speed.
Add both flours and mix some more. Expect a sticky dough – knead for 20-30 minutes. Resist the temptation to add excess flour; just enough to form a tight ball.
For bulk fermentation, transfer the dough to a lightly oiled bowl, cover it with plastic wrap or a damp towel, and let it rest in a warm spot.
After bulk fermentation is done, transfer the dough into the fridge. The purpose of this is to ensure the dough is cold and hard because it will shape better this way.