Preheat the oven to 350F/180C
Grease a 9" pan and dust with flour, set aside.
In a large bowl combine room temperature egg, sugar and vanilla and beat with an electric mixer until the mixture is thick, pale and is tripled in volume. It will take between 7 and 10 minutes depending on the power of your mixer.
Combine flour and baking powder and add to the egg mixture through a flour sifter in three additions. Gently and carefully fold in flour with a spatula after each addition, making sure not to deflate the batter or the sponge will not rise. (Read the recipe notes in the main body of the post)
Carefully fold in chopped rhubarb with a spatula and put in the prepared pan.
Bake for 40-45 minutes until golden and the cake tester comes out clean.
Dust with powdered sugar before serving.