To make a delicious rhubarb cake, mix 1½ cups sugar with ½ cup softened butter, then beat in 1 egg and 1 tsp vanilla. Stir in 1 cup buttermilk, then fold in a mix of 2 cups flour, 1 tsp baking soda, and ½ tsp salt. Add 2 cups chopped rhubarb. Pour the batter into a greased 9x13-inch or deep-dish pie pan. For a crumb topping, combine ½ cup brown sugar, 1 tsp cinnamon, and 2 tbsp butter, then sprinkle over the top. Bake at 350°F (175°C) for 45–50 minutes until golden and a toothpick comes out clea