In a large bowl, whisk together flour, cocoa powder, cornstarch, baking powder, 1 teaspoon salt and nutmeg until well combined.

In a separate large bowl using an electric hand mixer or in the bowl of a stand mixer with the paddle attachment, beat granulated sugar and butter on medium speed just until mixture appears sandy, about 1 minute.

Beat in egg yolks on medium speed until mixture is light and thickened, about 1 to 2 minutes.

Add flour mixture in three additions, alternating with sour cream, beating after each addition until batter is well-combined.

Cover bowl with plastic wrap; refrigerate at least 1 hour, up to 4 hours, until batter is slightly firm.

Meanwhile, heat oil in a large pot or deep saucepan with deep fry thermometer with a clip attached to the sides. Heat oil to 325°F.

On a very well-floured surface using a well-floured rolling pin, roll dough out to ½-inch thickness.

Use a well-floured or two different-sized biscuit cutters to cut dough into approximately 16 to 18 donuts and donut holes (re-roll scraps once, if needed).

Fry donuts in batches, about three at a time, 2 minutes each side or until puffed and cooked through.

Use a slotted spoon or deep fry skimmer to transfer cooked donuts to a paper bag or paper towel-lined baking sheet to drain excess oil. Repeat with remaining donuts and donut holes.

Next, make the glaze: In a small saucepan over low heat, heat milk to just below a simmer (when it begins to steam).

Remove from heat; whisk in powdered sugar, pinch salt and vanilla until smooth.

Use a fork to carefully dunk donuts one at a time into glaze, coating both sides.

Lift to allow excess glaze to drip off donut, then transfer to a cooling rack with parchment or paper towel underneath to catch excess glaze.

Allow donuts to sit until glaze is fully set, about 15 to 20 minutes.

Donuts are best served the day they're made, but will keep in an airtight container at room temperature for up to 2 days.