Make the topping: In a small bowl combine sugars and cinnamon; stir well. Set aside.
Preheat oven to 325. Grease an 8x8" baking dish.
Using an electric mixer mix flour, sugar, salt, & baking soda together. Add butter & mix. Mixture will become crumbly looking & a little dry. Add egg, egg yolk, yogurt/buttermilk, & vanilla. Beat until light & fluffy.
Pour mixture into greased baking pan using a rubber spatula to smooth it out.
Add cinnamon topping by sprinkling it over the top.
Bake for 30-35 minutes, until toothpick inserted in center comes out clean.
Let cool for 30 minutes
Recipe by THE SEMISWEET SISTERS at https://www.thesemisweetsisters.com/2020/08/21/coffee-cake/