Preheat oven to 350°F and coat a 9 x 13-inch baking pan with nonstick cooking spray.
Chop rhubarb, reserve 1/2 cup, and toss the remaining with 2 tablespoons of flour.
Cream butter, sugar, egg, and vanilla together. Add buttermilk. Finally, add the dry ingredients; flour, salt, and baking soda. Fold in rhubarb and spread into prepared pan. Sprinkle the topping, along with reserved 1/2 cup chopped rhubarb, over the cake.
Bake for about 40 minutes, or until a cake tester inserted in the middle comes out clean.