• Heat oven to 350 degrees F.  Line a 9x13-inch baking dish with parchment paper, then butter the paper and set aside.

Strawberry Rhubarb Filling

• In a medium-size saucepan over medium heat, combine the 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 the brown sugar, flour, and cinnamon.  Using a pastry cutter, cut the butter into the sugar and flour mixture until it resembles coarse sand.  Use your hands to squeeze handfuls of topping together, creating some larger clumps, then set aside.

Cake Batter

• In the 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 the eggs, one at a time, scraping the bowl and beating well between each addition.  Mix in the vanilla.
• In a 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 ½ 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 top of the fruit filling, using a knife or spatula to gently spread it to the edges as well.
• Evenly sprinkle the crumb topping over the cake batter, squeezing handfuls of topping again to make sure there are at least some larger chunks of topping. 
• Transfer the coffee cake to the oven and bake for 50 to 60 minutes, until a cake tester inserted into the center of the cake comes out clean.  Cool completely.