Preheat your oven to 190C and line 1 ½ muffin trays (18 holes) with paper cases.

Cut the butter into small cubes and place into a microwave safe bowl. Cook for 30 second spurts on high until it has just melted. Set aside to cool slightly.

Roughly chop the Mars Bars and set aside until needed.

Sift the self raising flour and cocoa powder into a large bowl and stir through the caster sugar.

Add the milk, eggs and vanilla into a small bowl and whisk together.

Pour the milk mixture along with the cooled melted butter into the bowl with the dry ingredients. Gently mix until the ingredients have almost combined.

Add the chopped Mars Bar pieces and stir to combine.

Divide the mixture between the muffin cases and place into the oven to bake for 18 - 20 minutes.