Preheat the oven to 180°C (160°C for fan forced ovens). Line 2 x 12-hole muffin pans with paper cupcake cases.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Make up the cake mix as directed on the box using the eggs, water and oil. Divide the mixture evenly between the cupcake cases. Bake both pans together in the oven for 18-22 minutes. Remove from the oven and carefully transfer the cupcakes to a wire rack to cool completely; about 30 minutes.
To decorate, spread 300g of the frosting over the cupcakes and sprinkle with the biscuit crumbs to resemble soil. Put 350g of the remaining frosting in a medium bowl and colour with the orange food colouring paste or gel. Put the remaining 150g of frosting in a small bowl and colour with the green food colouring paste or gel.
Spoon the orange frosting 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 frosting into a piping bag fitted with a writing nozzle (about 2mm).
Pipe 3 orange frosting carrots, as though growing out of the soil, onto each cupcake. For the carrot ‘tops’, pipe the green frosting 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.