Lightly spray pan with cooking spray. Line with 2 crisscrossed pieces of parchment, leaving a 2-inch overhang. Spray parchment.
In a saucepan, combine butter, marshmallows, sugar, milk, and salt. Cook over medium-high heat, stirring frequently, until marshmallows have melted, 6-7 minutes. Stir in vanilla. Remove from heat; stir in chocolate until melted. Pour mixture into prepared pan. Let cool for 30 minutes, then refrigerate until set, at least 1 hour and up to 3 days.
Using an overhang, lift fudge onto cutting board; cut into 1-inch squares. With gloved hands, to prevent fudge from becoming too slippery and messy, roll into balls (if mixture is very cold, it may crumble a bit - just let it warm slightly first), then roll in desired toppings as you go.
Refrigerate until firm for 1 hour, or store in refrigerator in a single layer on a parchment-lined baking sheet wrapped in plastic for up to 3 days.