Strawberry Rhubarb Coffee Cake

i've never accually made this, but i've had it a couple times and it is deeeeelish.

Filling:
3 c. rhubarb (cut in 1/2" pieces)
2 c. strawberries, mashed
1 T. lemon juice
1 c. sugar
1/3 c. cornstarch

Cake:
3 c. flour
1 c. sugar
1 t. baking soda
1 t. baking powder
1 c. butter
1 t. salt
1 c. buttermilk
2 eggs
1 t. vanilla

Topping:
3/4 c. sugar
1/2 c. flour
1/4 c. butter

Prepare filling: cook rhubarb and strawberries for 5 mins. covered on medium heat. mix sugar and cornstarch and gradually stir into fruit. cook until thickened and glossey. cool. (this part can be frozen for later use.)

Prepare cake: mix flour, sugar, baking soda, baking powder. cut in butter and salt. beat together buttermilk, eggs, and vanilla. add flour/butter mix to buttermilk mix, stir to blend. grease 9x13, spread 1/2 cake mixture. cover with cooled filling. spoon remaining cake mixture over filling and carefully spread to cover.

Prepare topping: mix sugar and flour. cut in butter until it resembles coarse crumbs. sprinkle over cake.

bake 350 for 50 mins.