1) In a saucepan, mix the rhubarb sauce, cornstarch, honey, orange peel and juice, salt and lemon juice together.
2) Simmer over a very low heat, stirring continuously, until the mixture is clear and thick.
3) In a bowl with the egg yolk, stir a little hot mixture, then return into the saucepan.
4) Allow the mixture to simmer for another 2 minutes.
5) Turn off the heat and stir in margarine; allow to cool.
6) Use the filling to spread between 2 layers of cake.