Place sugar in a small pot and heat to 240F (117C).

Add the egg whites to the bowl of a stand mixer. When the sugar reaches 230F (111C), add the cream of tarter and start whisking the egg whites on high speed.

When the sugar reaches 240F (117C), turn the mixer to slow speed and drizzle the hot sugar syrup into the bowl, then turn mixer back to high and continue to mix until the mixture has cooled down.

Once the mixture has cooled, add the butter cubes, one at a time. Wait for each piece to be fully incorporated before adding the next.

The buttercream appears to curdle but will come back once all the butter is incorporated.

Use immediately or refrigerate until required.

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