In a large bowl, whisk together the flour, baking powder, baking soda, nutmeg, and salt.
In another bowl, beat together the sugar, melted butter, eggs, buttermilk, and sour cream until smooth.
Add the wet ingredients to the dry and stir until just combined. The dough will be sticky.
Cover and chill the dough for at least 1 hour to make it easier to handle.
On a lightly floured surface, roll out the dough to about 1/2-inch thickness.
Cut out doughnuts with a floured doughnut cutter or two round cutters.
Heat oil in a large, heavy pot to 350°F (175°C).
Fry the doughnuts in batches, turning once, until golden brown (about 1-2 minutes per side).
Remove with a slotted spoon and drain on paper towels.
Dust with sugar or cinnamon sugar while still warm, if desired.
Serve fresh and enjoy!