Mix softened butter and sugar until light and creamy.
Mix in egg yolk one by one.
Fold in flour.
Spread evenly on a baking tray with baking paper.
Cut rhubarb in about 1 cm pieces and spread on base.
In a fat-free bowl, beat the egg whites with the sugar and salt until stiff.
Fold in coconut flakes.
Spread coconut mix evenly over the rhubarb.
Bake in preheated oven at 180° C for about 30 minutes.
Leave to cool and cut in pieces.
These coconut rhubarb bars taste best on the next day. (Cover so that it does not dry out).