Grease an 18cm X 28cm rectangular slice pan.
Line base and sides with baking paper leaving a 2cm overhang.
Stir butter and chocolate in a saucepan over low heat until melted and smooth. Cool slightly.
Stir in sugar and egg.
Add sifted flours, cocoa and biscuits.
Stir until combined.
Spoon into prepared pan.
Beat cream cheese and butter in a small bowl with electric mixer until smooth.
Add remaining ingredients.
Beat until combined.
Spread evenly over chocolate mixture.
Cook in a moderately slow oven (160 degrees) for 20 minutes or until set.
Melt remaining chocolate either using a double boiler (or in the microwave!).
Drizzle over slice.
Refrigerate until totally cool and set.
Cut into squares.