Preheat oven to 350ºF. Butter and flour an 8” round spring-form pan.
In a large bowl whisk flour, sugar, baking powder and salt.
In another bowl beat egg, yogurt, melted butter and vanilla until fluffy. Stir into flour mixture until blended. Batter will be stiff. Stir in rhubarb. Spread into prepared pan and smooth top.
Combine brown sugar, flour and cinnamon. With your fingers work together until crumbly. Sprinkle over cake batter.
Bake 45-50 minutes.
Cool. Serve with ice cream.