Wash and peel the rhubarb and cut into pieces approx. 1 cm in size.
Mix the eggs well with the powdered sugar in a mixing bowl. Add sifted flour and baking powder and stir everything into a liquid batter while adding milk and oil.
Generously butter a baking pan approx. 30 cm in diameter and pour in the dough. Spread the rhubarb on the dough and sprinkle with 1 tablespoon of sugar. Bake at 180 ° C for about 30 minutes
For the topping, whisk the egg with the powdered sugar and the very soft butter. Spread the icing on the cake and bake the cake for another 15 minutes.