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1
Preheat the oven to 190u00b0C (375u00b0F)/ 170u00b0C (340u00b0F) fan oven. Line one 12-hole muffin tray with cupcake cases and have another tin and a few more cases handy in case your mixture makes more (mine made 16).
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2
For the cupcakes, put the butter, caster (superfine) sugar, flour and baking powder in a bowl and whisk using a handheld electric whisk on a low speed (or you can make this in a freestanding mixer if you have one) until the mixture resembles breadcrumbs and looks fairly even in texture and colour.
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3
In a jug, whisk the milk, vanilla and eggs by hand to combine. Pour three-quarters of the liquid mixture into the dry ingredients and whisk on a low speed until smooth and thick. Add the rest of the liquid and 150g (5.5 oz.) of the dulce de leche/caramel and whisk in until the batter is smooth. Make sure to scrape down the sides of the bowl so that all the ingredients are mixed in thoroughly.
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4
Divide the mixture between the cupcake cases, filling each about 2/3 full. If you have too much mixture for 12 cases, then use your spare tin and extra cases to fill a few more. Bake in the oven for 16-20 minutes (mine took 16 minutes in a 170u00b0C fan oven). They will be well risen and springy. Cool for a few minutes in the tin and then transfer to a wire rack to cool completely.
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5
While the cakes are cooling, make the icing. Melt the chocolate in a bowl over a pan of barely simmering water. Set aside to cool slightly.
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6
Put the butter in another bowl and sift in the icing sugar and cocoa powder. Beat until blended and even. Add the cooled melted chocolate, vanilla and double cream and whisk until smooth and creamy. This should take a minute or so. Set aside while you fill the cupcakes.
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7
Use a knife to cut a small cone out of the top of each cupcake (cut a circle about 2cm diameter with a knife inserted about halfway into the cake, at an angle pointed towards the middle). Fill the hole with about 1 teaspoon of the remaining dulce de leche and then put the top back on, pressing it down. You won't get it completely flat, and the caramel might ooze out a bit at the top, but you can cover this up with the icing!
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8
Once you have filled all the cupcakes, ice them using either an icing bag fitted with a 1M tip (or any other you like), or just use a knife if you prefer. Top each cupcake with a slice of Mars bar. Now decide if you want to share them or if you just want to eat them all yourself.
