Preheat your oven to 350°F (175°C).

In a stand mixer bowl, mix together the all-purpose flour, sugar, baking powder, baking soda, salt, and nutmeg on low speed until evenly combined, about 1-2 minutes.

In a separate mixing bowl, whisk together the Greek yogurt, milk, eggs, sourdough discard, melted butter, and vanilla extract until smooth.

Pour the wet ingredients into the bowl of dry ingredients and mix on low speed just until combined; avoid overmixing.

Lightly grease your donut pans with coconut oil or cooking spray.

Using a piping bag or a cookie scoop, evenly divide the batter among the donut molds, filling each about three-quarters full.

Bake for 14-16 minutes, or until golden brown and a toothpick inserted comes out clean.

Allow the donuts to cool in the pan for about 5 minutes, then transfer to a wire rack for another 5 minutes.

Place the powdered sugar into a zip-top bag, then gently shake each donut inside to coat them evenly.