In a large saucepan combine rhubarb, strawberries and lemon juice. Cover and cook over medium heat for about 5 minutes. Combine sugar and cornstarch; stir into strawberry rhubarb mixture. Bring to a boil over medium heat, stirring continuously until thickened; remove from heat.

In a large bowl, combine flour, sugar, baking powder, baking soda and salt. Cut in butter with a pastry blender until mixture resembles coarse crumbs.

In a separate bowl, beat together buttermilk, eggs and vanilla; stir into crumb mixture. Spread half of the batter evenly into a greased 9x13x2-inch baking dish. Carefully spread filling over the batter. Drop remaining batter evenly over filling with a tablespoon.

TOPPING:

In a small saucepan over low heat, melt butter. Remove saucepan from heat; stir in flour and sugar until mixture is crumbly then sprinkle over batter. Lay foil on lower rack to catch any juicy fruit spillovers.

Bake at 350°F for about 45 minutes, or until cake is finished.

Cool cake in pan on rack. Cut rhubarb coffee cake into squares to serve.