In a large mixing bowl, whisk together the cake mix, oil, milk, eggs, and sour cream until fully combined.
Transfer batter to a piping bag.
Spray a donut pan with non-stick baking spray (the kind with flour works best). Pipe a ring of batter into each donut cavity. Do not fill each cavity more than ½ full.
Bake donuts for 10 minutes, until puffed and just beginning to turn golden.
Remove from oven and turn out onto a cooling rack. Continue baking batches of donuts.
Prepare the glaze by whisking together powdered sugar, milk, and cocoa powder, if using.
Dip the smoothest side of the donut into the glaze, allowing some excess to drip off and then place on a cooling rack or platter. Decorate immediately with sprinkles before the glaze sets.
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Notes
Storage:Keep leftover donuts for up to 3 days in an airtight container. Place in a single layer, separate with parchment paper if needed.Notes:These donuts taste more like cupcakes, even though the sour cream gives them a moist and denser texture.Donuts should be cooled before dipping in the glaze so it's doesn't run off. You can also drizzle it over if needed. Make sure the glaze is somewhat thin. The thicker it is the harder it is to dip and keep the donuts together.Add sprinkles within a couple minutes of dipping, or else they will not stick.Avoid filling the donut pan cavities more than ½ full. They will puff up and turn to an odd shape if there's too much batter.