Heat oven to 350 degrees. Spray a 9x13 pan with butter or a non-stick spray, set aside.

Filling: In a medium sauce pan over medium heat, combine rhubarb, strawberries, sugar, cornstarch, and water. Stir frequently until the cornstarch begins to thicken but the rhubarb is still fairly firm, about 5 minutes. Remove from heat and set aside.

Crumb topping: In a large bowl, combine brown sugar, flour, and cinnamon. Using a pastry blender, cut the butter into the mixture until it resembles course sand. Use your hands to squeeze handfuls of topping together, creating some larger clumps,then set aside.

Cake batter: In a bowl of a stand mixer, beat the cold butter until smooth and creamy. Slowly add the sugar while continuing to beat at medium speed, until light and fluffy. Add eggs one at a time, scraping the bowl and beating well between each addition. Mix in vanilla.

In separate bowl, whisk together the baking soda, baking powder, salt, and flour, then add the flour mixture to the batter 1 cup at a time, alternating with 1/2 of the sour cream between each addition and mixing on low speed just until everything is combined. Be careful not to over mix.

Spread half of the cake batter into the bottom of the prepared pan, using a spatula to make an even layer right to the edges of the pan. Top with the strawberry rhubarb filling, gently spreading that to the edges as well. Dollop the remaining cake batter over the top of the filling, using a knife or spatula to gently spread it to the edges.

Evenly sprinkle the crumb topping over the batter, squeezing handfuls of topping again to make sure there are at least some larger chunks of topping.

Bake 50-60 minutes until a knife inserted into the center comes out clean. Cool completely.