1. Feed sourdough starter until active and bubbly.

2. Whisk together warm milk, sugar, melted butter, egg, and vanilla extract.

3. In a separate bowl, sift flour, baking powder, salt, and ground cinnamon.

4. Add sourdough starter to wet mixture, then gradually add dry ingredients, mixing until a soft dough forms.

5. Knead dough until smooth, about 6–8 minutes, then cover and rest for 2–3 hours until slightly puffed.

6. Roll dough to 1/2-inch thickness, cut into rings, and place on a parchment-lined baking tray.

7. Proof donuts for 45–60 minutes until puffy and springy.

8. Preheat oven to 3506F (1756C) and lightly grease a doughnut pan.

9. Bake donuts for 10–12 minutes until golden and springy to the touch.

10. Mix granulated sugar and cinnamon in a shallow bowl.

11. Brush warm donuts with melted butter and roll in cinnamon sugar until evenly coated.

12. Cool donuts slightly on a rack and serve warm.