Step 1

In a large saucepan, combine rhubarb, strawberries and lemon juice. Cover and cook over medium heat 5 minutes. Combine sugar and cornstarch; stir into saucepan. Bring to a boil; cook and stir 2 minutes or until thickened. Remove from heat and set aside.

Step 2

Preheat oven to 350°. 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.

Step 3

Spread half the batter evenly into a greased 13x9-in. baking dish. Carefully spread filling on top. Drop remaining batter by tablespoonfuls over filling.

Step 4

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 oven rack to catch any juice spillovers.

Step 5

Place coffee cake on middle rack; bake 40-45 minutes. Cool on a wire rack.