Line a baking tray with non-stick baking paper. Heat a small pan of water until gently simmering and set a heatproof bowl over it. Add the marshmallows and butter spread and leave to melt, stirring often.
Remove the bowl of melted marshmallows from the heat and then pour in the rice pops. Gently stir to combine and keep going until well mixed. Lightly dampen your hands with water and scoop out a little of the mixture. Shape into an oval and place on to the prepared tray. Dampen your hands again and repeat with the remaining mixture until you have 16 in total.
Dip the handle of a teaspoon in water and then carefully press down along the centre of each ‘brain’ to create a groove. Use the red icing to drizzle squiggly lines or splat on the icing. Leave to set for 2 hours to firm up before serving.