Prepare a baking or cookie sheet with a large piece of parchment paper.
Pour both bags of white chocolate into a large glass bowl. Melt the chocolate in the microwave in 30 second intervals, stirring well after each time until white chocolate is completely melted smooth.
Pour rice krispies into chocolate and stir well. Pour mixture out onto a cookie sheet covered with parchment paper and spread out evenly.
Sprinkle M&Ms over the top of the chocolate and lightly press into the mixture. Let the bark sit in the refrigerator for several minutes until the chocolate is hard.
Remove from refrigerator and break into uneven pieces. Store bark in an airtight container.