Preheat the oven to 180°C conventional or 160ºC fan-forced.
Line a 12 hole muffin tray with paper cases. Sift the dry ingredients into a large bowl, then add the sugars and stir in. Add the egg, milk, oil and vanilla then beat with a mixer until combined. Scrape the sides of the bowl then add the boiling water and mix until just combined (the batter will be runny). Pour the batter into a measuring cup (it’s easier and cleaner than using a spoon) and pour evenly into each paper case until ¾ filled (don’t over fill or the mixture will overflow).
Place in the oven and bake for 20-22 minutes or until a tester inserted in the centre comes out clean.
Mix icing sugar with hot water. Add butter and beat until light and fluffy. Pipe or spread onto muffins. Freeze any left-over icing.