Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

Prepare the rhubarb filling: In a medium bowl, toss the chopped rhubarb with 1/2 cup sugar and 1 tablespoon flour. Set aside.

Prepare the streusel topping: In a separate bowl, combine 1 cup flour, brown sugar, and cinnamon. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Place the mixture in the refrigerator while preparing the batter.

Prepare the cake batter: In a large bowl, whisk together 2 cups flour, baking soda, and salt. In another bowl, cream the softened butter and 1 1/2 cups sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla.

Alternate adding the dry ingredients and the sour cream to the wet ingredients, beginning and ending with the dry ingredients, mixing just until combined. Do not overmix.

Assemble the cake: Spread half of the batter into the prepared pan. Sprinkle the entire rhubarb filling evenly over the batter. Carefully dollop the remaining batter over the rhubarb. Finish by sprinkling the chilled streusel topping evenly over the entire surface.

Bake for 50 to 60 minutes, or until a wooden skewer inserted into the center (avoiding rhubarb pockets) comes out clean. Let cool completely on a wire rack before slicing and serving.