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

Prepare the rhubarb filling: In a medium bowl, toss the chopped rhubarb with 1/4 cup sugar and 1 tablespoon flour. Set aside.

Prepare the streusel topping: In a separate small bowl, combine the 1/2 cup flour, 1/2 cup brown sugar, and 1/2 teaspoon cinnamon. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Refrigerate until needed.

Make the cake batter: In a large bowl, whisk together the dry ingredients (1 3/4 cups flour, baking powder, and salt). In a separate bowl, cream together the softened butter and 1 cup sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.

Alternate adding the dry ingredients and the buttermilk to the butter mixture, mixing until just combined. Be careful not to overmix.

Assemble the cake: Spread half of the batter evenly into the prepared pan. Sprinkle the rhubarb filling over the batter. Dollop the remaining batter over the rhubarb and gently spread to cover. Finally, sprinkle the chilled streusel topping evenly over everything.

Bake for 50 to 55 minutes, or until a wooden skewer inserted into the center (avoiding the rhubarb pockets) comes out clean. Let the cake cool in the pan for at least 30 minutes before slicing and serving.