Line a 9-by-13-inch baking pan with two sheets of waxed or parchment paper in a crisscross manner (one lengthwise, one crosswise) so ends overhang sides of pan; coat evenly with cooking spray.
In a heavy-bottomed saucepan over medium heat, cook sugar, salt, butter, cream, and marshmallows, stirring, until butter and marshmallows are almost
melted, 5 to 6 minutes.
Bring mixture to a boil; cook, stirring occasionally,
5 minutes. Remove from heat. Add chips and vanilla; stir until chips are melted. Pour mixture into lined pan.
Let fudge cool in the pan at room temperature,
3 hours. Use edges of paper to lift out fudge; place on cutting board, and remove paper. Cut out shapes with cookie cutters, or cut fudge into bars. Sprinkle evenly with crushed candy.
Fudge can be stored in an airtight container at room temperature up to 1 week.