Make the topping first. Mix flour, sugar, brown sugar, & cinnamon together in a bowl. Add melted butter & mix until dry crumb mixture forms. Place in the refrigerator.
Preheat oven to 350 degrees. Grease a 8x8" square baking dish.
Make the cake: Using an electric mixer, cream butter & sugar together. Add in egg.
In a separate bowl mix sour cream, yogurt, & vanilla together. In another bowl sift flour, baking powder, & baking soda together.
Add sour cream mixture to the butter/sugar & mix. Then add in flour mixture & mix until smooth.
Pour into prepared dish. Take the topping out of the frig & crumble between your fingers onto the top of batter.
Bake for 30-35 minutes or until toothpick inserted in center comes out clean.
Recipe by THE SEMISWEET SISTERS at http://www.thesemisweetsisters.com/2015/04/14/streusel-coffee-cake/