This is truly the queen of buttercreams: silky, buttery, light and airy, and a bit temperamental. Combining the Italian meringue with the butter is the tricky part. It is essential that the two mixtures have near the same temperature. And sooner or later it happens to everyone: Instead of becoming a beautifully emulsified satiny texture, it starts to curdle and separate. Your heart drops and panic sets in--all that expensive butter and time....But all is not lost. Here are some tips and also a solution should all else fail:
Use an instant read thermometer to ensure that the temperature of the mixture is between 65° to 70°F/19° to 21°C and adjust as needed. If not using a thermometer, try adjusting with just a small amount of the buttercream.
If all else fails, with your hands, squeeze out the liquid that has separated and pour it into a large measuring cup with a spout. On high speed, beat the remaining butter until it becomes smooth. Then gradually beat in the liquid. The resulting buttercream will be less airy but perfectly emulsified and silky smooth.
Note: You will have a higher degree of success if using high fat butter.
Also, it works best to add all the meringue to all the whipped butter rather than the reverse. This technique is detailed in Rose's Heavenly Cakes and The Baking Bible.