1. Heat oven to 375 degrees F. Coat 8 x 8 x 2-inch square baking pan with nonstick cooking spray.
2. Melt butter in saucepan. Remove pan from heat. Stir in flour, sugar and cinnamon until mixture becomes coarse crumbs. Measure out and reserve 1/2 cup of the crumbs.
3. Whisk baking soda and sour cream in small bowl. Whisk in vanilla and egg yolk. Stir sour cream mixture into crumb mixture in saucepan, stirring until nearly smooth, with only a few small lumps remaining. Spread batter in prepared pan. Sprinkle top with reserved 1/2 cup crumbs.
4. Bake in 375 degrees F oven 20 minutes or until golden brown and wooden pick inserted in center comes out clean. Serve warm or at room temperature. Or cover and freeze up to 1 month; thaw overnight in refrigerator. Sprinkle with powdered sugar before serving, if desired.