In a large mixing bowl, cream together the softened butter and brown sugar until well combined and lighter in color. Add the egg and beat until light and fluffy. Mix in the sourdough starter discard, vanilla extract, and grated fresh apple until well combined.
12 tablespoons (1 ½ sticks or 170 grams) unsalted butter, ¾ cup (150 grams) brown sugar, 1 large (50 grams) egg, ¾ cup (150 grams) sourdough starter discard, 2 teaspoons vanilla extract, ½ cup (75 grams) grated fresh apple
Alternate adding ⅓ of the dry ingredients and ½ of the apple cider (or apple juice), starting and ending with the dry ingredients. Mix until just combined, being careful not to overmix.
¾ cup apple cider or apple juice
Divide the batter evenly among the 12 donut cavities in the prepared pans.
Bake for 15-17 minutes, or until a toothpick inserted into the thickest part of a donut comes out clean. Remove from the oven and allow the donuts to cool in the pans for 10-15 minutes.
While the donuts are cooling, mix granulated sugar and ground cinnamon in a small bowl. Once the donuts have cooled slightly, remove them from the pans and place them one at a time into the cinnamon-sugar mixture, shaking gently to coat.
2 tablespoons granulated sugar, ½ teaspoon ground cinnamon
Transfer the coated donuts to a wire cooling rack. Serve warm and enjoy!
Notes
Storage: Allow any leftover donuts to cool completely before storing them in an airtight container at room temperature for 2-3 days. For longer storage, freeze for up to 2 months.