Preheat your oven to 180 degrees and grease and line the base and sides of a 20cm round cake tin with baking paper.
Put all of your ingredients into a large bowl and whisk until mixture is just smooth.
Add the chopped Mars Bars and carefully fold through.
Pour mixture into the cake tin and bake for approximately 40 minutes or until cake is cooked through when tested with a skewer.
Let the Chocolate Mars Bar Cake cool in the tin for 10 minutes before carefully transferring onto a wire rack to cool completely.
Once the cake has cooled, decorate with your favourite chocolate icing or ganache and decorate with the remaining Mars Bar.