Crackle-Topped Rhubarb Coffee Cake

To make cake: Preheat oven to 350 degrees F. Coat an 8-inch-square baking pan with cooking spray. Sift flour, 1 cup sugar, baking powder, 1 teaspoon cinnamon and salt into a medium bowl. Whisk together egg, buttermilk, oil and vanilla in a large bowl. Add the flour mixture and stir with a rubber spa