Preheat your oven to 350°F (175°C).

In a stand mixer bowl, combine the dry ingredients: all-purpose flour, sugar, baking powder, baking soda, salt, and nutmeg. Mix on low speed for about 30 seconds.

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

Gently incorporate the wet ingredients into the dry ingredients using your stand mixer. Mix on low speed just until blended—about 30 seconds.

Grease your donut pans thoroughly with non-stick cooking spray or melted butter. Fill each donut cavity about three-quarters full.

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

Cool in the pans for about 5 minutes, then transfer to a wire rack to cool completely.

While still warm, place powdered sugar in a zip-top bag, add donuts, seal, and shake to coat evenly.