Start by preheating your oven to a warm temperature that will help the donuts rise nicely. Line a cookie sheet with parchment paper if you prefer an easier cleanup.

In a mixing bowl, carefully whisk together the unbleached all-purpose flour, baking powder, baking soda, salt, and ground cinnamon.

In a separate bowl, mix the sourdough discard and sour cream or plain Greek yogurt until smooth.

Gradually add the granulated sugar to the sourdough mixture, stirring until it is fully incorporated.

Fold the dry flour mixture into the wet mixture until no dry flour is visible.

Scoop the batter onto the prepared cookie sheet, forming small rounds, leaving space between each for rising.

Brush the tops of the donuts with melted butter.

Bake the donuts in the preheated oven until lightly golden and a toothpick inserted into the center comes out clean.

While the donuts are baking, combine the sugar and ground cinnamon in a shallow dish.

Once done, remove the donuts from the oven and brush them with remaining melted butter.

Roll the warm donuts in the sugar-cinnamon mixture until fully coated.

Allow the donuts to cool slightly before serving.