While not a traditional coffee cake with the addition of rhubarb, this recipe does not disappoint! The perfect balance of moisture and flavor combine for a tart yet sweet treat!
Preheat oven to 350°F. Grease a 9x9" baking dish and set aside.
In a large bowl, combine the white sugar, brown sugar, 2 cups flour, baking powder, salt, and cinnamon.
Add the pieces of cold butter and cut them into the dry ingredients until the mixture resembles wet sand with pieces of butter the size of a pea or smaller. Remove 1 ¼ cups of the mixture and set aside.
To the remaining crumbled mixture left in the bowl, add the eggs, sour cream, and vanilla. Whisk until combined and pour into prepared baking dish. Add the chopped rhubarb on top, sprinkle with the remaining 1 tbsp flour. Top with reserved streusel.
Bake in preheated oven until golden and a skewer inserted into the middle of the cake comes out clean, 40-55 minutes. Remove from oven and cool 15 minutes before serving.