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 top.
Bake in the preheated oven for 30 to 40 minutes.
Beat the remaining 3 egg whites to soft peaks, then gradually add 150g caster sugar and beat till very stiff. Remove pie from oven and swiftly heap meringue 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.