Preheat oven to 400º. Combine flour, oats and brown sugar in bowl. Cut in butter with pastry cutter, two knives or your clean fingertips until the mixture resembles coarse crumbs. Stir in chopped nuts. Pat mixture into 9" X 13" pan and bake for 15 minutes. Stir with fork while still warm to crumble, then let cool completely.

Soften ice cream on counter until you can just stir it. Spread half the crumbles in an 8" X 8" pan. Drizzle half the butterscotch topping over crumbles. Carefully spoon and spread vanilla ice cream over topping. Drizzle with remaining topping and then with rest of crumbles. Gently press down and freeze a couple hours until firm. Cut into squares and serve.