Have you got a stand mixer?

It works fine. Don't use castor sugar (it will burn and you'll have gritty bits in your icing). You'll know the eggs are cooked when they are warm to touch, not hot. Warm. If they are hot they will crystalise. I have made this icing a billion times and never had any disasters if I follow this method. I use my KA and use the bowl for the KA over a pot of simmering water and stir gently with a whisk till all the sugar is dissolved. I just rub a bit of whites between my fingers to feel if it's dissolved. Then I put it onto the mixer and on about speed 8 I whisk till stiff peaks and then from there continue for 7 minutes, or until the outside of the bowl has completely cooled. Then I slowly add the butter with the flat beater a few cubes at a time. And if it appears to separate beat again at a high speed. And then on a slow speed to remove air bubbles. If it's clumpy you need to beat again at a high speed.

Sometimes I add all the butter sometimes I don't. So just do it to taste. And know it makes a lot of icing

This is one of my secret weapons