Preheat the oven to 180C, 350F, Gas Mark 4.
Beat the butter and sugar together until creamy.
Separate the egg whites and egg yolks.
Add the egg yolks to the butter and sugar, mixing after each one.
Add the flour, reserved rhubarb juice and milk to the mixture, whisking well.
Whisk the egg whites until they form soft peaks and stir into the mixture.
Pour the mixture over the top of the rhubarb in the oven proof dish.
Bake in the oven for 30 minutes until the top is golden and firm.