The trick to this is to freeze your mars bars at least an hour before you use it so I place mine in the freezer just before I head out for brunch with hubs.
For the cake, melt 375grms of dark chocolate with 250grms of butter. I basically break up the chocolate (as I used a block instead of buttons or chips) & place in a microwave safe bowl with butter cutted in cubes. Microwave for a minute to melt & then keep stirring will combined & mixed. All the stirring should cool the mixture down.
Stir into the mixture 6 eggs, one at a time & a teaspoon of vanilla extract. At this stage, you can add a cup of sugar if you like but I chose to omit the sugar as a trial & it turned out well (& not as sweet).
Once the eggs are mixed in, stir in 2 cups of self raising flour & a cup of chocolate powder. You want your cake batter to be a good consistency so if it is too runny, add a bit more flour or if it is too dry, add a bit more milk.
Pour batter into cake tin or silicon mould. Then cut up the Mars bars & place randomly into mixture leaving gaps between each lot. I usually use 2-3 full sized Mars bars (each bar cut into 5-6 pieces) or about 8 snack sized Mars bars (each bar cut into 3).
Then place cake in oven for 45 minutes.
Using another 250grms of butter, whisk in high & when the butter is whisked well & turned a paler colour, add to it 1 cup of icing sugar, a cup of chocolate powder & half a cup of milk. Keep whisking till combined and if a lil runny, you may need to add a bit more icing sugar. This is the buttercream for icing the cake.
Once the cake is cooked, allow it to cool in the tin/mould for about half an hour (remember, Mars bars contains caramel & nougat so that will make the middle runny when hot & the cake will collapse of taken out too early). Then place it on a plate or wired rack to cool for a further hour.
Once cooled, you can start icing the cake with the buttercream.