4 (9 inch) square, heavy-duty plastic freezer bag; spoon filling into bag. Freeze until firm.
1 cup butter
1/2 cup packed brown sugar
1 cup white sugar
1 egg
1 1/2 cups peanut butter
6 ounces white chocolate chips
To make the filling: Melt butter in a large bowl. Mix together brown sugar, white sugar and egg; stir into brown sugar mixture.
To make the chocolate filling: Heat oven to 350 degrees F (175 degrees C) Line a 9x13 inch pan with parchment paper. Butter the bottom of the pan, about 1/4 inch thick, and sprinkle with white chocolate chips.
Bake in preheated oven for 1 hour, or until brown sugar is set and chocolate filling has begun to brown. Remove paper and ice cream while still in plastic bag; remove and chill until serving.