Chunks of rhubarb accent this moist, tender cake. Sprinkle cinnamon and sugar over the batter before baking to create a crisp top.
To make Cake: Preheat oven to 350F. Oil lightly an 8-inch square baking pan or spray with nonstick cooking spray. Set aside.
Sift flour, sugar, baking powder, cinnamon and salt into mixing bowl. Whisk together egg, buttermilk, oil and vanilla in a separate bowl. Add flour mixture to egg mixture, and stir with a rubber spatula or wooden spoon just to blend. Stir in rhubarb. Turn batter into prepared pan, spreading evenly.
To make Topping: Stir together sugar and cinnamon. Sprinkle evenly over top of cake.
Bake for 45 to 50 minutes, or until top is brown and crackled and a toothpick inserted in center comes out clean. Remove from oven, and cool in pan on a wire rack for 15 minutes. Cut into squares, and serve while still warm.