Preheat oven to 190 C / Gas 5. Grease a 23cm springform tin.
Beat butter and 150g sugar till foamy. Add the egg yolks and 1 egg white, set the other egg whites to the side. Sift flour and baking powder and add. Mix till smooth and tip into the springform tin.
Trim the ends of the rhubarb and slice. Spread on the base and bake in the preheated oven for 30 to 40 minutes.
Beat the remaining 3 egg whites with 150g caster sugar till very stiff and heap onto the rhubarb filling. Put back in the oven and bake until the meringue is no longer sticky and lightly browned, about 15 further minutes.