Place two shortbread biscuits onto a plate.
Mix mascarpone, icing sugar, and nuts in a small bowl.
Spoon a quarter of the mixture onto each biscuit and cover with a second biscuit.
Repeat the layering with the remaining mixture and top with a final biscuit.
Chill the shortbread stacks in the refrigerator for 10 minutes.
Gently heat orange juice and caster sugar in a saucepan until dissolved.
Add rhubarb and simmer for 6 to 8 minutes until soft.
Add chopped orange pieces and heat through for 2 minutes.
Stir in the chopped fresh mint.
Drizzle the hot rhubarb sauce over the shortbread stacks to serve.