Prepare a baking tray with grease proof paper and spray with cooking spray.
Crush the peppermints inside a plastic bag with a rolling pin until broken into tiny pieces.
Place white chocolate in a heatproof bowl set over (not in) a saucepan of simmering water.
Heat, stirring occasionally, until smooth, 4 to 6 minutes.
Remove from heat; stir in rice cereal.
Transfer mixture to prepared tray; with a spatula and spread evenly to the edges of tray.
Sprinkle with the crushed candy; with a baking paper covering the entire surface, press in gently (paper prevents hands from sticking to candy and chocolate).
Chill until firm, 20 to 30 minutes.
Do not leave any longer in the refrigerator.
Peel waxed paper off.
Break into 2-inch pieces and wrap for gifts or serve.