In a saucepan, combine sugar and cornstarch. Stir in rhubarb and strawberries. Bring to a boil over medium heat; cook for 2 minutes or until thickened. Remove from the heat; stir in lemon juice, cool.
For Cake, combine flour, sugar, baking powder and baking soda in a large bowl. Cut in butter until mixture resembles coarse crumbs. Beat the eggs, buttermilk and vanilla. Stir into crumb mixture just until moistened. Spoon two-thirds of the batter into a greased 13 x 9 x 2 inch baking pan. Spoon cooled filling over batter. Top with remaining butter.
For topping, combine sugar and flour. Cut in butter until mixture resembles coarse crumbs, sprinkle over batter. Bake at 350 degrees for 45-50 minutes, or until golden brown. Cool on wire rack.