Make the topping first. In a small mixing bowl mix flour, sugar, brown sugar, & cinnamon together. Add melted butter & mix until dry crumb mixture forms. If it's not very crumbly add in a little extra flour.
Preheat oven to 350 degrees. Grease 1-2 donut pans.
Make the cake batter: In a mixing bowl stir butter & sugar together. Add in egg. Mix sour cream & vanilla in. In another bowl sift flour, baking powder, & baking soda together.
Add flour mixture to the sour cream/liquid mixture & mix until smooth.
Spoon into prepared donut pans, about 2 tablespoons per donut. Take the crumb topping and crumble between your fingers onto the top of each donut.
Bake for 14-16 minutes or until tops are starting to turn a slight golden brown and donut is springy to the touch.
Recipe by THE SEMISWEET SISTERS at http://www.thesemisweetsisters.com/2018/01/10/coffee-cake-baked-donuts/