In a large microwave-safe bowl, combine semisweet chocolate chips, sweetened condensed milk, and unsalted butter.

Microwave the mixture in 30-second intervals, stirring well between each, until fully melted and smooth.

Once melted, pour the mixture into an 8x8 casserole dish lined with parchment paper and spread it evenly.

Before the fudge begins to set, sprinkle Christmas colored M&M candies over the top and press them lightly into the fudge.

Place the dish in the refrigerator and chill until set, approximately 2 hours.

Once set, lift the fudge out using the parchment paper and slice into squares with a sharp knife.