Place the chewy toffees into a large microwave-safe bowl with the cocoa powder and butter. Microwave on high (900 watts) for 3 minutes. Carefully remove the bowl from the microwave using oven gloves. Alternatively, melt the toffees with the cocoa powder and butter in pan over a gentle heat. Stir the mixture with a wooden spoon, then tip in the marshmallows (there is no need to stir again at this stage).
Microwave on high (900 watts) or return the pan to the heat for 1 minute, then stir quickly to mix. Add one-third of the rice crispies and mix them into the toffee mixture. Add the rest of the rice crispies in 2 batches, stirring well after each addition.
Grease a 10 cm (4 inch) ring (rum baba) mould. Take a handful of the rice crispie mixture and press it into the mould to make a ring shape. Press it down firmly with the palm of your hand, then twist and upturn it to release it from the mould. Place it on a sheet of nonstick baking paper. Repeat with the rest of the mixture to make 10 rings in all. To decorate, press white bows on to the mixture (optional). Allow to set before serving.