In small microwavable bowl, microwave peanut butter uncovered on High 15 to 20 seconds or until lightly melted and somewhat pourable. In large bowl, mix ice cream and peanut butter, using spatula or electric mixer, until almost no streaks of peanut butter remain. Stir in marshmallows. Work quickly so ice cream does not completely melt. Spread evenly over crust. Cover with plastic wrap, pressing against ice cream to help prevent ice crystals from forming. Freeze 4 to 6 hours.