Line an 8×8-inch pan with foil and spray with cooking spray.
In a microwave-safe bowl, combine chocolate chips, sweetened condensed milk, and butter.
Microwave on high for 1 minute, then stir until chips are almost melted.
Continue microwaving in 30-second increments, stirring between, until mixture is smooth and glossy.
Remove from microwave, stir in vanilla extract, and set aside 1 cup of the mixture.
Fold mini marshmallows into the remaining fudge mixture, then spread into the prepared pan.
Drizzle reserved fudge on top and use a knife to create a swirl pattern.
Refrigerate for 2 hours until firm, then lift from foil and cut into squares.
Notes
During prep, I lined the pan and found that spraying foil helped release fudge cleanly after chilling.I learned that stirring off the heat immediately once the chips begin melting ensures a glossy, silky texture without graininess.In my third storage test, the fudge remained tender at room temperature through five days, though I recommend refrigeration for optimal freshness.