Preheat the oven to 350 degrees F and lightly mist a donut pan with non-stick spray.

Place the flour, cocoas, baking soda, and salt in a large bowl and whisk to combine.  

In a smaller bowl, mix the sour cream, sugar, oil, egg, and vanilla together.  

Pour the wet ingredients into the dry, and fold together just until combined (batter will be thick and lumpy).  

Transfer the batter to the prepared pan and bake the donuts for 12 to 14 minutes, or until a toothpick inserted in the thickest part of a donut comes out clean or with a few moist crumbs.  

Cool on a wire rack, then glaze and coat with chocolate crumb topping.

Stir the powdered sugar, milk, and vanilla together in a medium bowl.

Melt the butter in a medium pot over low heat.  

Add the flour, sugar, cocoa, and salt, and toss together with a fork until crumbly.  

Press the crumb topping into the glaze while still wet.