Whisk flour, cocoa, sugar, baking powder, baking soda, salt, and nutmeg in a large bowl.
In another bowl, whisk milk, sour cream, egg, vanilla, and melted butter until smooth.
Stir wet ingredients into dry just until combined; the batter will be thick. Rest 5 minutes.
Heat 2–3 inches of oil to 350°F in a deep pot, keeping a thermometer clipped to the side.
Scoop heaping tablespoons of batter and gently drop into the oil. Fry 2–3 minutes, turning for even browning.
Transfer to a rack set over a sheet pan to drain. Keep oil near 350°F between batches.
Whisk powdered sugar, milk, vanilla, salt, and corn syrup until smooth and pourable.
Dip warm donut holes into the glaze, letting excess drip off. Set on the rack to firm up.
Notes
Try stirring 1 tsp instant espresso into the glaze for a mocha twist, or dust warm donut holes with cinnamon sugar instead of glazing. Store leftovers airtight at room temperature up to 2 days; refresh 10 seconds in the microwave to soften.This recipe is an original creation inspired by classic Glazed Chocolate Donut Holes flavors. All ingredient ratios and instructions are independently developed.