Preheat the oven to 350F, butter your donut pan and sprinkle with cocoa powder, tapping out the excess. Set aside.

In the bowl of a stand mixer, beat together the butter and sugar until light and fluffy.

Add in the egg, cocoa, and vanilla, and beat well.

Mix in the sour cream and baking soda.

Mix in the flour and salt until incorporated, then slowly pour in the milk.

Fill your donut molds about ⅔ full {I found the best way to fill them is to pour the batter in a large ziploc bag, snip the corner, and pipe it in. Fast and mess-free!}.

Bake for about 15 minutes, until a toothpick inserted in the thickest part of the donut comes out clean.

Allow to cool for a few minutes in the pan, then turn donuts onto a cooling rack and allow to cool completely.

When ready to glaze, heat the white chocolate in a microwave-safe bowl in 30 second intervals until completely smooth. Slowly whisk in the heavy cream until a smooth, creamy glaze forms.

Dip the tops of the donuts in the glaze, then top with sprinkles as desired. Glaze will remain sticky and not harden. Enjoy with cold milk! :)