Grease an 8”-x-8” baking pan with cooking spray. In a medium saucepan over medium heat, melt butter, peanut butter, vanilla, and salt.
Remove from heat and stir in chocolate chips. If necessary, return to low heat to finish melting chips. Let cool to room temperature.
Stir in marshmallows. Pour mixture into pan and refrigerate until set, 3 hours. Cut into squares.
