Line a 9×13‑inch pan with parchment, leaving overhang. Mist with non‑stick spray.
In a double boiler, melt butter and white chocolate chips, stirring until smooth.
Off heat, fold in half the marshmallows until partially melted, then add remaining marshmallows and stir until the mixture is cohesive. Microwave 20–30 seconds if needed.
Quickly fold in gumdrops.
Press mixture evenly into prepared pan. Cover and chill at least 4 hours or overnight.
Lift slab onto cutting board. Slice into 1‑inch squares with a warm knife.
Notes
- Fresh gumdrops prevent hard, sugary bits in the final candy.
- Swap gumdrops for fruit‑slice candies or chopped gummy bears for a fun twist.