Preheat oven to 400 ° with the rack in the middle. Butter an 8×8-inch baking dish.
Mix together the water, cornstarch, and 1/2 cup sugar in a small saucepan, then add the rhubarb. Bring to a simmer, stirring constantly, and simmer for 3 minutes. Remove from heat.
In a separate bowl, whisk together the flour, baking powder, salt and remaining 1/2 cup of sugar.
In another bowl, whisk together the egg, buttermilk, melted butter and vanilla.
Add the wet ingredients to the dry ingredients and whisk just until combined into a thick batter.
Reserve 1/2 cup of the rhubarb mixture. Pour the remaining rhubarb mixture into the bottom of the baking dish and spoon the batter evenly over it. Drizzle the remaining rhubarb mixture over the top of the batter. Bake until a tester inserted into the middle of the cakes comes out clean, 25-30 minutes. Cool on a rack slightly before serving.