Melt the milk chocolate in a heatproof bowl set over a pan of barely simmering water, stirring occasionally until the chocolate has completely melted.
Remove from the heat and use a wooden spoon to gently mix the Rice Krispies into the milk chocolate, ensuring they are all evenly coated.
Use a spoon to fill the cupcake cases with chocolate krispies, then gently insert a small wooden teaspoon into each crispy cake. Transfer to the fridge for 30 minutes to set.
Meanwhile, melt the white chocolate in a heatproof bowl as before. Once set, remove the crispy cakes from the fridge and add a teaspoon of the melted white chocolate to the top of each. Decorate with sprinkles and return to the fridge to set.
Remove the crispy cakes from the fridge 30 minutes before serving and discard the cupcake cases.