Finely grate the zucchini and place it in a cheesecloth or a clean kitchen towel. Squeeze out as much moisture as possible to avoid overly wet donuts.

In a large bowl, whisk the flour, sugar, baking powder, baking soda, cinnamon, nutmeg, and salt until well combined.

Whisk together the sour cream, buttermilk, egg, melted butter, and vanilla extract in a separate bowl until smooth.

Gently fold the grated zucchini into the wet ingredients.

Combine Dry and Wet Ingredients: Pour the wet ingredients with the zucchini into the dry ingredients. Gently fold until just combined. Be careful not to overmix.

Spoon or pipe the batter into the prepared donut pan, filling each cavity about 3/4 full

Bake for 11-12 minutes until the donuts are golden, and a toothpick inserted into the center comes out clean.

Let the donuts cool in the pan briefly before transferring them to a wire rack to cool slightly.

In a small bowl, mix the sugar and cinnamon. Melt the butter in a separate bowl.

While the donuts are still slightly warm, brush each with a small amount of melted butter, then immediately dip and roll them in the cinnamon sugar mixture until fully coated.