Transfer puffed rice cereal to a bowl and set aside. Add milk chocolate to a heatproof bowl and set over a pot of simmering water. Melt chocolate while stirring occasionally. Remove from heat and let cool down to room temperature. Add melted chocolate to puffed rice cereal and stir to combine. Transfer the mixture into a parchment paper-lined baking dish. Refrigerate for approx. 2 hrs.