Preheat Oven to 400 degrees Fahrenheit.
In a medium saucepan, Add rhubarb pieces, lemon juice, sugar and cornstarch. Simmer at medium temperature, stirring constantly for 5 minutes. Let cool to room temperature.
Prepare a cast iron skillet by buttering and flouring the bottom and edges.
In a large bowl, sift together flour, sugar, baking powder and salt. Set aside.
In a medium bowl, whisk together butter, egg, vanilla, sour cream and milk. Pour wet ingredients into bowl with dry ingredients and stir until incorporated.
Spread about 2/3 of the rhubarb mixture evenly into the skillet. Pour batter over the top. Dollop remaining rhubarb on top of batter.
Using a knife, gently swirl the fruit topping into the batter. Bake about 25 minutes or until a toothpick inserted into the center removes cleanly. Do not over bake.
Let cool in the pan on a wire rack. Spoon into dishes and garnish with whipped cream before serving.