Preheat the oven to 350°F (177°C). Grease a donut pan with nonstick spray. This recipe makes 8 donuts, so grease 2 cavities of a second donut pan or bake in batches. Set aside.

In a large bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.

In a medium bowl, whisk together the egg, brown sugar, milk, and sour cream/yogurt until smooth. Add the melted butter and vanilla, whisking until fully combined. Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix. The batter will be thick.

Transfer the donut batter to a gallon-size zip-top plastic bag and use scissors to snip off the tip of one bottom corner. Pipe the batter into the donut pan cavities, filling each about 1/2–2/3 full.

Bake for 9–10 minutes or until the donuts spring back when lightly pressed. Cool the donuts in the pan set on a cooling rack for 2 minutes, then remove them from the pan and place on the rack to slightly cool before coating. Re-grease the pan and bake the remaining donut batter.

Combine the sugar and cinnamon in a medium bowl. Dip the donuts in the melted butter, then dunk in the cinnamon-sugar mixture, coating all sides.

These donuts taste best served shortly after coating. Cover leftover donuts tightly and store at room temperature for 1–2 days or in the refrigerator for up to 1 week.