Heat oven to 180C/160C fan/gas 4. Tip the sugar, flour, cocoa and baking powder into a large bowl. Whisk together the eggs, vegetable oil and milk, then stir together with the dry ingredients until well combined. Add the milk chocolate chips. Place cupcake cases into a 12-hole bun tin. Divide the mixture between the cases, then bake for 20 mins until risen and cooked through. You can now leave to cool and store for up to 2 days in an airtight container.
Just before serving (either warm from the oven or cold), arrange marshmallows over the tops of the cakes. Heat grill to medium and pop the cakes under it for 30 secs, watching all the time, just until the marshmallows are lightly browned. Remove and eat straight away.