Whilst the rhubarb is cooking, combine the butter and remaining portion of castor sugar in a heat proof bowl sitting on a pan of simmering water. Stir gently for 10 minutes until the butter has melted and the mixture has thickened a little. Thoroughly whisk the egg and egg yolk and add to the bowl, continue to stir for a further 15 minutes to allow the curd to cook and thicken to the consistency of custard. Remove from the heat and whilst it is cooling, blend the cooked rhubarb until smooth. Combine both mixtures and allow to cool (this should take no longer than 50 minutes to ensure it is refrigerated within the hour).