Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Line 2 x 12-hole muffin trays with paper cupcake cases.
Make, bake and cool the cake mix as directed on the box for cupcakes using the oil, water and eggs.
To decorate, use 300g of the icing to spread over the cupcakes and sprinkle with the biscuit crumbs to resemble soil. Put 350g of the remaining icing in a mixing bowl and colour with the orange food colouring paste or gel. Put the remaining 150g of icing in a separate mixing bowl and colour with green food colouring paste or gel.
Spoon the orange icing into a piping bag fitted with a round nozzle (about 6mm in width), or simply fill a disposable piping bag and snip off the end leaving a roughly 6mm hole. Spoon the green icing into a piping bag fitted with a writing nozzle (about 2mm).
Pipe 3 orange icing carrots, as though growing out of the soil, onto each cupcake. For the carrot ‘tops’, pipe the green icing in strands on top of each carrot. Serve immediately and store any leftover cupcakes in an airtight container in a cool place for 2-3 days.