1. 1. Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, the first 1/2 cup of granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

2. 2. Mix Wet Ingredients: In a separate medium bowl, lightly beat the large egg. Then, add the active sourdough discard, milk, melted unsalted butter, and vanilla extract. Whisk these wet ingredients together until they are well combined and smooth.

3. 3. Combine Wet and Dry: Pour the wet ingredient mixture into the dry ingredient mixture. Stir with a spoon or spatula until just combined. Be careful not to overmix; a few lumps are perfectly fine and will result in a tender donut. Overmixing can lead to tough donuts. The dough will be thick and sticky.

4. 4. Prepare for Frying: In a large, heavy-bottomed pot or Dutch oven, pour enough vegetable oil to reach a depth of about 2-3 inches. Heat the oil over medium-high heat until it reaches 350-375 F (175-190 C). If you don’t have a thermometer, you can test the oil by dropping a tiny piece of dough into it; if it sizzles and floats to the top immediately, the oil is ready.

5. 5. Fry the Donuts: Using a spoon or a small ice cream scoop, carefully drop spoonfuls of the dough into the hot oil. Do not overcrowd the pot; fry only 3-4 donuts at a time to maintain the oil temperature. Fry for 1-2 minutes per side, or until golden brown and cooked through. They will puff up nicely as they cook.

6. 6. Coat with Cinnamon Sugar: While the donuts are frying, prepare the cinnamon sugar coating. In a shallow dish or a Ziploc bag, combine the remaining 1/2 cup granulated sugar and 1 tablespoon ground cinnamon. Once the donuts are golden brown, remove them from the oil with a slotted spoon or spider and place them on a paper towel-lined plate to drain excess oil for about 30 seconds. Immediately transfer the warm donuts to the cinnamon sugar mixture and toss gently to coat thoroughly. Serve warm and enjoy!