Lay a sheet of filo pastry on to a baking sheet covered with baking paper, brush with melted butter and sprinkle over a little caster sugar. Cover with another 3 sheets of filo, each brushed with butter and sprinkled with sugar, press down.
Cut into even rectangles, you will need 12 rectangles in total (3 for each portion)
Bake at 160C for 8-10 minutes until golden brown, leave on the tray for a few minutes, then transfer to a cooling tray until cold.
Measure the length of the pastry rectangles, then cut your rhubarb into lengths to match. You will need 24 lengths of rhubarb (6 for each portion). Place the rhubarb into a baking dish and sprinkle over the remaining sugar (75g), roast the rhubarb in the oven at 160C for around 12 minutes or until you can easily pierce the stalks with a knife but before the rhubarb starts to disintegrate. Remove and leave to cool.
Whip the double cream and until very thick and stiff, fold in the custard and load the mixture into a piping bag with a large star nozzle (Wilton 1M).
Place the pastry rectangles onto the serving plates, pipe cream in three rows, place three stalks of rhubarb on top, then place another pastry on top and repeat the cream and rhubarb. Top with the final pastry rectangle and dust with icing sugar.