Clean the bowl of your stand mixer, whisk attatchment, paddle and handwhisk with a paper towel dipped in white vinegar. This helps remove traces of fat which can make your egg whites deflate.

Add your caster sugar and egg whites to the bowl and place it over a saucepan with simmering water making sure the bottom of the bowl does not touch the water.

Keep whisking as the mixture heats up to ensure that the sugar granules are completely dissolved in the egg whites.

Remove the bowl from the heat and place it on the mixer. With the whisk attachment whisk the mixture till it is cooled completely. This is meringue.

Add the cubes of butter piece by piece waiting until one piece is fully incorporated till you add the next. Don't rush this process. The mixture may start to curdle or look soupy. Don't worry keep whipping, it will come together.

Add vanilla and whip.

Switch to the paddle attatchment or if using a hand mixer fold in with a spatula. Do this till you see no more bubbles but just silky smooth buttercream.

You can flavor with anything from fresh fruit to mixed nuts. Experiment with your favorite flavors.