Preheat the oven to 350 degrees F, and lightly grease a non-stick donut pan.
Place the flour, baking soda, salt, and nutmeg in a medium mixing bowl, and whisk to combine.
Place the sour cream, sugar, oil, egg, and vanilla in a large liquid measuring cup, and stir together until smooth.
Add the liquid ingredients to the dry, and gently fold together until just barely combined (it's ok if there are a few lumps).
Transfer the batter to the prepared pan, and bake the donuts for 12 to 16 minutes, or until just beginning to brown around the edges. Cool for 20 minutes, then remove from the pan.
GLAZE
In a medium mixing bowl, stir the powdered sugar, milk, and vanilla together until smooth.
Dip the tops of the donuts in the glaze, allowing the excess to drip off.
Place the donuts on a wire rack set over a baking sheet, and allow the glaze to dry.
Notes
If making muffins or mini-muffins, bake time could vary.