In a small bowl, heat the 1/3 cup butter and chocolate chips in the microwave for 30 seconds. Remove from microwave, stir, and keep heating in 15 second intervals until mixture is completely melted and smooth.
Stir the 2 cups of crushed oreos into the melted chocolate chip mixture, and spread in the bottom of a 9" pie pan. Let cool completely.
In a large bowl, beat cream cheese until light and fluffy. Slowly add it condensed milk and peanut butter, and continue mixing until smooth. Stir in lemon juice and vanilla, and then fold in whipped cream.
Pour peanut butter mixture into chocolate crust and place in freezer. Freeze for about 4 hours, until it is solid.
Before serving, drizzle with fudge sauce.
Notes
If it's getting too soft after serving, you may need to stick it back in the freezer.