Preheat oven to 350 degrees F. Soften butter, and use an electric mixer to cream butter and white sugar until light in colour and fluffy. Add egg and vanilla and mix until creamy looking.
In a separate bowl, mix flour and baking soda together. Then, mix with butter mixture alternately with buttermilk.
Cut rhubarb into 1' pieces and mix into batter with a spatula. Scrape batter into a buttered 9" by 13" glass or ceramic dish.
Mix brown sugar, flour, cinnamon, and melted butter together in a bowl and sprinkle over cake batter. Bake 50 minutes until a toothpick comes out clean. Cool 15-20 minutes before serving.