Make the dough

• Combine the flour, sugar, sourdough starter, yeast, milk, eggs and salt in a bowl.
• Knead until the dough comes together, then work in the softened butter until smooth and elastic.

Let the dough rise

• Cover the dough and leave it to rise until doubled in size.

Shape the donuts

• Roll the dough to 1.5 cm thick.
• Cut 9 cm circles and cut a hole in the center of each.

Proof the donuts

• Place the donuts on a tray and let them rise until puffy.

Make the blueberry glaze

• Blend the blueberries and strain the purée through a sieve.
• Whisk the powdered sugar with the blueberry purée and lemon juice until smooth.

Fry the donuts

• Heat the oil to 350°F (175°C).
• Fry the donuts until golden on both sides, then drain.

Glaze and serve

• Dip the warm donuts in the blueberry glaze and let the excess drip off.