1.Preheat the oven to 180C/350F/Gas 4. Grease and line two 20cm/8in sandwich tins.
2.For the cake, place all of the cake ingredients, except the boiling water, into a large mixing bowl. Using a wooden spoon, or electric whisk, beat the mixture until smooth and well combined.
3.Add the boiling water to the mixture, a little at a time, until smooth. (The cake mixture will now be very liquid.)
4.Divide the cake batter between the sandwich tins. Break the mars bar in to four pieces and bury them in the cake batter in both sandwich tins. Bake in the oven for 25-35 minutes, or until the top is firm to the touch and a skewer inserted into the centre of the cake comes out clean.
5.Remove the cakes from the oven and allow to cool completely, still in their tins, before icing.
7.To assemble the cake, run a round-bladed knife around the inside of the cake tins to loosen the cakes. Carefully remove the cakes from the tins on a serving plate and then eat.
225g/8oz plain flour
350g/12½oz caster sugar
85g/3oz cocoa powder
1½ tsp baking powder
1½ tsp bicarbonate of soda
2 free-range eggs
250ml/9fl oz milk
125ml/4½fl oz vegetable oil
2 tsp vanilla extract
250ml/9fl oz boiling water
One large Mars Bar