Add the butter to the Thermomix bowl and mix for 40 sec/speed 5 until soft. Scrape down the sides of the Thermomix bowl with a spatula.
Insert the butterfly whisk: Add half of the icing sugar to the softened butter and whip for 1 min/speed 3.
Scrape down the sides of the Thermomix bowl, add the remaining icing sugar and 2 tablespoons of milk, and mix for 1 min/speed 3 until smooth and creamy.
Notes
StorageButtercream keeps well when stored in an airtight container in the fridge for up to one week. If you plan to use it sooner, it can be kept at a cool room temperature for up to one day.Buttercream can also be frozen for up to three months. Thaw overnight in the fridge, then bring to room temperature and re-whip before using.