Feed your sourdough starter at least 4 hours before starting the dough.

In a large mixing bowl, combine your bubbly sourdough starter, bread flour, eggs, sugar, and warm milk to form a sticky dough.

Allow the dough to rest for 30 minutes, then knead in salt and softened butter until smooth and elastic, about 10 minutes.

Transfer to a greased bowl, cover, and let rise at room temperature for 3-4 hours until expanded by one-third, then refrigerate overnight.

Remove from the fridge, let sit for 30 minutes, roll out to ½ inch thick, cut into 12 pieces, shape into balls, and proof for 30-45 minutes.

Heat oil to 360°F (182°C) and fry donuts for 2-3 minutes on each side until golden brown.

Cool slightly, optionally roll in sugar, and fill with desired filling using a piping bag.

Serve and enjoy your filled sourdough donuts!