Line an 8-inch or 9-inch square pan with foil. Spray lightly with cooking spray.
Combine chocolate chips and sweetened condensed milk in a large, microwave-safe bowl.
Heat on high for 30 seconds, stir, repeat until chocolate is melted and smooth.
Stir in vanilla, marshmallows, and pecans.
Pour into prepared pan and let cool to room temperature before transferring to refrigerator to chill.
Remove from pan, peel off foil, and cut into squares.
Store in an airtight container.
