Place the butter and vanilla together in a large bowl or stand mixer. If you are using a hand-held electric whisk, then add the icing sugar little by little as you whisk so that the icing sugar does not fly everywhere, and whisk well between additions.
If you are using a stand mixer, fit it with a paddle attachment, then add the icing sugar in one go and carefully place a tea towel over the machine, taking care not to get it anywhere near the paddle. Start mixing slowly at first, and then beat it for a good 5–7 minutes. Remove the tea towel when the icing sugar is all mixed in.
When the buttercream is super light and fluffy, use as desired. It keeps at room temperature for 3 days, or up to 2 weeks in the fridge covered with cling film. It freezes really well – defrost it at room temperature or overnight in the fridge (not in the microwave), then beat it again until light and fluffy before using.