Whisk sour cream, powdered sugar and 2 T whipping cream in bowl to blend. Add peanut butter and whisk until smooth. Spoon 1 T peanut butter mixture into each graham cracker crust; freeze 3 hours. Bring half C cream to simmer in heavy saucepan. Reduce heat to low. Add chocolate; stir until melted. Cool completely, stirring occasionally. Spoon 2 teaspoons chocolate mixture over each peanutbutter pie. Place in freezer until set. (can be prepared 1 week ahead. Keep frozen.) Using tip of small sharp knife as an aid, gently pry pies from tins. Let stand at room temperature for 10 minutes before serving.