In a saucepan combine rhubarb, strawberries and lemon juice. Cover and cook over medium heat for 5 minute. Combine sugar and cornstarch, stir into saucepan. Bring to a boil, stirring constantly until thickened, remove from heat and set aside. In a large bowl combine flour, sugar, baking powder, baking soda and salt.

Cut in butter until mixture resembles coarse crumbs. Beat buttermilk, eggs and vanilla, stir into crumb mixture. Spread half of the batter evenly into a greased 13x9 baking pan, spread filling on top drop remaining batter by tablespoonsfull over filling.

For topping melt butter in a saucepan, stir in flour and sugar until mixture resembles coarse crumbs, sprinkle over batter. Bake at 350 for 40-50 minutes