1. Preheat your oven to 350 F (175 C). Lightly grease a donut pan with non-stick cooking spray or butter to prevent sticking.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

3. In a separate medium bowl, whisk together the milk, sourdough discard, egg, melted butter, and vanilla extract until well combined.

4. Pour the wet ingredients into the dry ingredients. Stir gently with a spoon or spatula until just combined. Be careful not to overmix the batter; a few small lumps are perfectly fine. Overmixing can lead to tough donuts.

5. Spoon the batter into the prepared donut pan, filling each cavity about two-thirds full. An easy way to do this without mess is to transfer the batter to a large zip-top bag, snip off a corner, and pipe it into the molds.

6. Bake for 10-12 minutes, or until the donuts are golden brown and spring back when lightly touched. A toothpick inserted into a donut should come out clean.

7. While the donuts are baking or cooling, prepare the glaze. In a medium bowl, whisk together the powdered sugar, maple syrup, milk, and vanilla extract until smooth and free of lumps. If the glaze is too thick, add a tiny bit more milk; if too thin, add a little more powdered sugar.

8. Once the donuts are out of the oven, let them cool in the pan for 5 minutes before transferring them to a wire rack. While still slightly warm, dip the top of each donut into the maple glaze. Let any excess drip off, then place them back on the wire rack for the glaze to set.