Step #1 Topping: Preheat oven to 350°F In a greased, 9-inch heavy nonstick skillet, melt butter over med-heat/flame; stir in the brown sugar.
Step #2 Remove skillet from heat; spread sugar mixture evenly over bottom of skillet.
Step #3 Scatter rhubarb [cut in 1 inches pieces] evenly over sugar mixture; set skillet aside.
Step #4 Cake: In a medium bowl, baking powder, whisk together flour, & baking soda.
Step #5 In a separate bowl & using an electric mixer, beat together sugar, butter, & lemon rind until well mixd.
Step #6 Add egg & vanilla, beat well.
Step #7 Stir half of flour mixture into sugar mixture; stir in the buttermilk.
Step #8 Add remaining flour mixture, stirring just until mixd.
Step #9 Spread batter carefully over rhubarb in skillet; bake for 45 to 50 mins, until cake springs back when carefully pressed in center & a toothpick inserted in center comes out clean.
Step #10 As soon as cake comes out of oven, run a spatula around sides to loosen cake.
Step #11 Let cool in skillet for 10 mins.
Step #12 Invert cake on a plate.
Step #13 Serve warm or at about room temp with whip cream or ice cream.