Mix the flour, butter, sugar, baking powder and egg together. Press the dough into a springform pan, pull up the edge. Peel the rhubarb, cut into pieces and place in the form. Mix the sour cream with sugar and pudding powder well and pour over the rhubarb. Bake the cake at 190 degrees for about 45 minutes (intensive heat, so the base becomes crispy and the topping stays moist).