Preheat the oven to 180 degrees, and line a 12 hole muffin tray with paper cases.
Place the butter, caster sugar, self raising flour, eggs, milk and vanilla extract into your Thermomix bowl and mix for 15 seconds, speed 5.
Scrape down the sides of the bowl and mix for a further 10 seconds on speed 4 or until smooth and creamy.
Spoon mixture into the cupcake tray and bake for 18 -20 minutes or until golden and cooked through.
Allow to cool completely on a wire rack
Once cooled, to make the icing, combine the icing sugar ingredients into a small bowl. Add the water a tablespoon at a time and mix until it's just spreadable.
Top the cupcakes with the icing and decorate with M&M's.