Grease and lightly flour two baking tins and use a 5cm cutter to mark 12 evenly spaced circles (6 on each tray) as a guide for piping.
Beat the butter and icing sugar together until light and fluffy. Add in the flour and cornflour and mix together until just combined.
Split the mixture in half and colour half with red food colouring.
Place the mixtures into two separate piping bags and then cut large holes in the end and squeeze the mixtures down side by side into another piping bag fitted with a medium sized star nozzle. This assures that the mixtures are split 50/50 in the bag.
Pipe 12 two tone roses by starting in the centre of the circle and swirling around outwards to form a rose.
Place in the oven and bake for 10 minutes at 210°C/190°C fan or until lightly golden.
Meanwhile, beat together the butter, icing sugar, custard powder, vanilla and milk until you have a smooth and light buttercream. Place in a piping bag fitted with a rose nozzle.
Once the biscuits are baked, remove from the oven and allow to cool on the tray.
Once cool, pipe buttercream in circles on the underside of 6 biscuits, leaving room to spoon a little rhubarb preserve into the centre. Spoon in the rhubarb and then sandwich together with another biscuit. Repeat until all 6 biscuits are filled and sandwiched.
Transfer to a board to serve.