Preheat the oven to 350°. Grease a 9x13 pan and set it aside.
In the bowl of a mixer beat the butter, sugar, and brown sugar on medium speed until light and fluffy. Add in the egg, vanilla, and lemon zest and beat until incorporated.
In a medium bowl sift the flour, baking soda, and salt together.
In 2-3 additions, alternate adding the buttermilk and dry ingredients to the wet ingredients in the mixer. Once well combined, mix in the chopped rhubarb by hand. Pour the batter into the pan and top with the cinnamon-sugar topping.
Bake for 40-45 minutes, or until a knife comes out clean from the center. Serve warm.