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 spatula or wooden spoon just to blend. Stir in rhubarb. Turn the batter into the prepared pan, spreading evenly.

To prepare topping: Stir together 1/4 cup sugar and 1/2 teaspoon cinnamon in a small bowl. Sprinkle evenly over the top of the cake. Bake until the top is brown and crackled and a cake tester inserted in the center comes out clean, 45 to 50 minutes. Let the cake cool in the pan on a wire rack for 15 minutes. Cut into squares and serve warm.