Mix graham cracker crumbs and butter in a small bowl. Press crumb mixture on the bottom and up the sides of 24 mini-muffin tins (each about 4 cm in diameter). Place in freezer for 20 minutes.
Whisk sour cream, powdered sugar and 2 tablespoons whipping cream in a bowl to blend. Add peanut butter and whisk until smooth. Spoon 1 tablespoon of peanut butter mixture into each graham cracker crust; freeze 3 hours.
Bring half cup cream to simmer in a heavy saucepan. Reduce heat to low. Add chocolate; stir until melted. Cool completely, stirring occasionally.
Spoon 2 teaspoons of the chocolate mixture over each peanut-butter pie. Place in freezer until set. (can be prepared 1 week ahead. Keep frozen.) Using the tip of small sharp knife as an aid, gently pry pies from tins. Let stand at room temperature for 10 minutes before serving.