In a microwave safe bowl, mix together syrup and butter. Microwave on high for 1 minute until butter is melted and syrup is bubbly. Remove from microwave and stir in peanut butter then add rice crispies. Mix well.
Press 3/4 of the mixture on the bottom of a 13” x 9” pan. Freeze until set. Spread ice cream on top of crust and top with remaining mixture, spreading evenly as you can. Freeze until firm or after about 2 to 3 hours. Cut into squares to serve.