Preheat the oven to 190°C/fan 170°C/gas mark 5.
Mix together the Fate low protein chocolate flavour cake mix, margarine and ProZero.
Melt Vitabite in a heatproof bowl over a pan of boiling water.
Add the Vitabite to the mixture and mix well.
Pour into a lined 21cmx21cm baking tin and cook in the oven for 35 minutes.
Remove when cooked and allow to cool.
Stir green food colouring into the frosting then spoon into a piping bag with a rosette nozzle. Pipe icing onto the cool brownies to resemble grass.
Decorate with jelly beans or other permitted sweets.
ProZero, Fate low protein chocolate flavour cake mix and Vitabite are foods for special medical purposes and must be used under medical supervision.