Line your pan. Cover a large rimmed baking sheet with parchment paper, or clear a stretch of counter and line that instead. Do this before anything else — once the chocolate is melted, you will not have time to go looking for parchment.
Combine the dry mix. In your largest mixing bowl, gently toss together the Rice Chex, popcorn, pretzels, mini marshmallows and holiday M&M's. Use a bowl bigger than you think you need; the coating cannot reach the bottom layer if the mix is packed to the rim.
Melt the white chocolate. Melt the wafers according to the package directions, stirring between short bursts until completely smooth. White chocolate scorches faster than dark, so stop while a few unmelted pieces remain and let the residual heat finish the job.
Coat the mixture. Pour the melted white chocolate over the snack mixture and fold gently with a silicone spatula until everything is evenly coated. Fold rather than stir — the marshmallows and pretzels break down if you work it too hard.
Add the sprinkles right away. Scatter the sprinkles over the mixture while the chocolate is still wet. This is a thirty-second window; once the coating starts to set, nothing else will stick to it.
Spread and let it set. Turn the mixture out onto the parchment and spread it into an even layer. Let it cool completely at room temperature, about 30 minutes, until the chocolate is firm to the touch.
Break into clusters. Break the slab into bite-sized clusters with your hands and serve. Aim for clusters, not crumbs — a few pieces holding together is what makes it feel like a treat rather than cereal.