FILLING: In a large saucepan combine rhubarb, strawberries and lemon juice. Cover and cook over medium heat about 5 minutes. 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 13" x 9" x 2" baking dish. Carefully spread filling on top. drop remaining batter by tablespoonfuls over filling. FOR TOPPING: Melt butter in a saucepan over low heat. Remove from heat; stir in flour and sugar until mixture resembles coarse crumbs. Sprinkle over batter. Lay foil on lower rack to catch any juicy fruit spillovers. Bake at 350 degrees, for 40-45 minutes. Cool in pan. Cut into squares. Yield: 16-20 servings.