Line base of a 23cm loose-based flan pan with baking paper. Process broken biscuits into fine crumbs. Add melted butter and extra tablespoon of milk and process until well combined. Press mixture over base and sides of pan. Chill while making filling.
Place peanut butter and remaining milk in a small saucepan and heat on low for 10 minutes, stirring, until smooth. Remove from heat and cool.
Using an electric mixer with paddle or V-groove beaters, beat cream cheese with sugar until smooth. Stir in peanut butter mixture. Spoon into chocolate crust, cover loosely with plastic wrap and chill for three hours, until firm.
Place melted chocolate in a small plastic bag and snip off corner. Drizzle melted chocolate finely back and forth over cheesecake. Return to fridge and leave to set for 15 minutes. Cut into wedges to serve.