In a large bowl, mix together flour, sugar, cornstarch, salt, and nutmeg.

In another bowl, whisk sour cream, butter, and egg yolks until well combined.

Mix your wet and dry ingredients until the dough comes together and is evenly mixed. Cover the dough and chill in the refrigerator for an hour.

While the dough is chilling, make the glaze: whisk the powdered sugar, hot water, vanilla extract, and salt until smooth. Set aside.

When the dough has chilled for an hour, prep your space for frying: set a deep saucepan on the stove over medium-high heat and pour oil to a 2-inch depth. Insert a thermometer, if using.

Set a baking sheet next to the pan lined with a wire rack and place the bowl of glaze near the baking sheet.

On a floured surface, roll out dough to a ½ inch (1.5cm) thickness and cut circles with a 3-inch (7.5cm) cutter. Reroll and cut and scraps.

With a 1-inch (2.5cm) cutter, cut a hole in the center of the dough circles to form your doughnut shape.

When the oil has reached 325°F (165°C), fry 2-3 doughnuts or 4-5 holes at a time. Flip after a few minutes, once golden.

When both sides are golden brown, remove from the oil, drain for a few seconds on the rack and then dip the hot doughnuts in the glaze.

Return to the rack to let the glaze set and repeat with the remaining doughnuts and holes.

Doughnuts are best the day they are made, but you can store any leftovers in an airtight container at room temperature for up to two days.