1. Combine crushed crackers, 1/3 cup sugar, and melted butter in a medium bowl. Press mixture on bottom and up sides of an ungreased 9-inch deep-dish pieplate.
2. Bake at 350° for 8 minutes or until golden brown. Let cool on a wire rack.
3. Stir together chocolate candy bar pieces and 3 Tbsp. warm whipping cream until chocolate is melted and smooth. Pour into cooled piecrust.
4. Beat 2/3 cup whipping cream at medium-high speed with an electric mixer until soft peaks form.
5. Beat together cream cheese and confectioners sugar at medium speed with an electric mixer until blended. Add peanut butter, 1/3 cup whipping cream, 1 tsp. vanilla, and white chocolate, beating at medium speed until blended. Fold in 2/3 cup whipped cream. Spread mixture evenly over milk chocolate layer in pan. Cover and freeze 30 minutes or until slightly firm.
6. Spread Ganache evenly over chocolate layer, and sprinkle with chopped nuts.
7. Beat remaining 1 cup whipping cream at high speed with an electric mixer until soft peaks form. Add 1/4 cup confectioners sugar and remaining 1/2 tsp. vanilla, beating until stiff peaks form.
8. Spoon cream mixture into a 1-qt. zip-top plastic bag. (Do not seal.) Snip a tiny hole in 1 corner of bag, and pipe onto edges of pie by gently squeezing bag. Cover and chill at least 1 hour. Garnish, if desired.