Prepare the Pan
Line an 8×8-inch baking dish with parchment paper, leaving an overhang on two sides for easy lifting. Lightly butter the parchment and set aside.
Melt the Base
In a large microwave-safe bowl, combine the butter, white chocolate chips, and mini marshmallows. Microwave on high for 1 minute, then stir well using a lightly greased spatula. Continue heating in 30-second intervals, stirring thoroughly each time, until the mixture is fully melted and smooth.
Add the Festive Mix-Ins
Allow the mixture to cool for about 1 minute so the candies don’t melt. Gently fold in the chopped gumdrops until evenly distributed.
Set the Candy
Transfer the mixture to the prepared pan and quickly press it into an even layer using the greased spatula.
Chill Until Firm
Cover tightly with plastic wrap and refrigerate for at least 4 hours, or overnight for best results, until completely set.
Slice & Serve
Lift the candy from the pan using the parchment overhang. Place on a cutting board, peel away the paper, and cut into small squares or bars. Store in an airtight container in a cool place.