Make the Filling

1. Combine rhubarb, strawberries and lemon juice in a saucepan.
2. Mix sugar and cornstarch together and stir into fruit mixture.
3. Cook over medium heat.

4. Bring to a boil, stirring constantly until thickened and cook 15-20 minutes.
5. Remove from heat.

Make the Cake

1. Preheat oven to 350°F.
2. In a large bowl, combine flour, sugar, baking powder, baking soda and salt.
3. Cut in butter until mixture resembles coarse crumbs.
4. In a separate bowl, whisk together buttermilk, eggs, and vanilla.
5. Stir wet ingredients into crumb mixture.
6. Spread half the batter into a greased 9x13-inch baking pan.
7. Spread strawberry-rhubarb filling evenly over batter.
8. Drop remaining batter evenly over filling.

Make the Topping

1. Stir in flour and sugar until crumbly.
2. Cut in butter until mixture resembles coarse crumbs.
3. Sprinkle topping evenly over batter.

Bake

1. Bake for about 45 minutes or until golden brown and set.
2. Cool on a wire rack before cutting into squares.

Tip: Serve warm with whipped cream or vanilla ice cream for an extra-special summer dessert.