1. Preheat oven to 160°C. Grease a 4cm-deep, 22.5cm x 28cm (base) slab pan. Line with baking paper, allowing 2cm overhang at both long ends.
2. Combine butter and dark chocolate in a large, heatproof, microwave-safe bowl. Microwave, uncovered, on MEDIUM-HIGH (70%) for 3 to 4 minutes, stirring every minute with a metal spoon, or until almost melted. Add cocoa and whisk until smooth.
3. Add sugar and mix well. Add one third of the egg and stir to combine. Repeat with egg in 2 batches. Sift flour over chocolate mixture. Stir gently until just combined. Pour mixture into prepared pan. Sprinkle Mars Bars over batter and press in slightly.
4. Bake for 60 to 65 minutes or until a skewer inserted into the centre has moist crumbs clinging. Cool completely in pan. Cut into squares. Serve with cream.