1. Preheat oven to 350 degrees. Butter a 9-by-13-inch baking dish; set aside. In a medium bowl, whisk together flour, baking soda, and salt; set aside. In a liquid measuring cup, combine buttermilk and vanilla; set aside.

2. In the bowl of an electric mixer or using a handheld mixer, beat butter with 1 cup sugar until light and fluffy, about 2 minutes. Add egg, and beat to combine. Add the flour mixture, alternating with the buttermilk mixture, and starting and ending with the flour mixture. Stir in rhubarb.

3. Spread batter evenly into prepared baking dish. In a small bowl, stir together remaining 1/4 cup sugar and cinnamon. Sprinkle cinnamon sugar evenly over batter. Bake until a cake tester inserted in center of cake comes out clean, about 35 minutes.

4. Let cool on a wire rack in pan for 30 minutes before serving.