Preheat oven to 350º F (180ºC).
In bowl, toss crumbs with butter. Pat into 9-inch (23 cm) pie plate, without covering rim. Bake in centre of oven until firm, about 8 minutes. Let cool.
In bowl, whip cream; set aside. In large bowl, beat cream cheese until smooth; beat in peanut butter, butter and vanilla. Beat in icing sugar until fluffy. Fold one-quarter of the whipped cream into filling; fold in remaining cream. Spread over crust. Cover loosely and refrigerate until firm, about 2 hours.
Meanwhile, chop chocolate finely; place in heatproof bowl. In small saucepan, warm cream over medium-low heat until bubbles form around edge; pour over chocolate and stir until melted. Let cool. Drizzle over filling; sprinkle with nuts.
Cover loosely and refrigerate until set, 2 hours. (Make-ahead: Refrigerate for up to