In small bowl, beat brown sugar, 1/2 cup peanut butter, and the butter until light and fluffy. Beat in flour until crumbly. Sprinkle crumb mixture into ungreased 13 x 9-inch glass baking dish. Reserve half of crumb mixture (about 1 cup) for topping. Lightly press remaining crumb mixture into bottom of baking dish. Bake 10 to 15 minutes or until golden brown.
In small bowl, beat cream cheese, sugar, 1/2 cup peanut butter, and vanilla until smooth and creamy. Add 1 egg at a time, beating well after each. Fold in whipped cream. Pour over crumb mixture in baking dish.
In 1 quart saucepan, melt chocolate chips over low heat, stirring frequently, until melted and smooth. Drizzle chocolate over cream cheese mixture. With table knife, gently cut through both mixtures to marble. Sprinkle with reserved crumb mixture; press in slightly. Freeze at least 2 hours or until serving time. Let sit at room temperature 15 minutes before serving. Cut into squares.