Melt butter in medium saucepan. Add marshmallows; cook and stir on low heat until marshmallows are completely melted. Remove from heat; immediately stir in 2-1/4 cups of the cereal. Firmly press cereal mixture onto bottom and up side of 9-inch pie plate sprayed with cooking spray. Cool.
Spread ice cream into crust; cover. Freeze 2 hours or until firm.
Remove pie from freezer about 10 min. before serving. Let stand at room temperature to soften slightly. Sprinkle with the remaining 1/2 cup cereal and drizzle with warmed peanut butter before cutting into eight wedges to serve.