Pre heat oven to 220°C / 430°F / 200°C fan-forced and line a 12 hole muffin pan with 6 cupcake papers, using every second hole. See notes
In a large mixing bowl, whisk together the cooled melted butter, oil, sugar, eggs, buttermilk and vanilla.
In another bowl, combine the flour, baking powder and soda, salt and nutmeg.
Add the dry ingredients to the wet mixture, along with the chocolate chips.
Lightly fold through the mixture (be careful not to overmix)
Rest the mixture on the bench for 15 minutes, to allow the batter to thicken.
Using a large ice cream scoop, place heaped scoops of mixture into the lined muffin pan. Be sure to fill the paper so the mixture is heaped on top.
Sprinkle the muffins with raw or demarara sugar and extra chocolate chips
Bake at 220°C for 7 minutes, then reduce the temperature to 180°C /350°F or 160°C fan-forced and bake for another 12-13 minutes.
Remove from the oven, rest muffins in the pan for 5 minutes, then remove them from the pan, onto a cake rack (or just eat them now!)
Muffins are best eaten on the day, but will keep for a couple of days in an airtight container