Add the chocolate and heavy cream to a heat proof mixing bowl and microwave for 1 minute, stir and then 30 seconds each time after that, mixing each time until smooth. Set aside to cool at room temperature.
Fill a large saucepan one-quarter of the way with water. Let it come to a gentle boil over a medium–high heat.
Separate your egg whites from your yolks in a large, very clean, glass or metal mixing bowl. We only need the egg whites for this recipe, so you can store the yolks in an airtight container in the fridge to use for something else. If you need ideas, there are loads on the internet, but my go-to recipe for using up egg yolks is French pastry cream.Yum!
Add your sugar to the egg whites to the mixing bowl and use a hand whisk to mix them together.
Place your bowl on top of the pan of boiling water, making sure the bottom of the bowl doesn’t touch the water. This is called the double-boiler method and is a very gentle way of cooking something or melting something.
Gently whisk your egg white and sugar mixture for about 3–4 minutes, or until the sugar is completely dissolved and you reach 160°C / 320°F. The best way to check that it’s dissolved is by running it through two fingers. If you can’t feel the sugar granules, then it’s time to take it off the heat. If you can feel the granules, continue whisking for another 2–3 minutes and keep testing until you can no longer any granules.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment and whisk on high speed for about4–5 minutes. The mixture will become thick and glossy and will begin to cool.
Gradually add the softened butter, 1tablespoon at a time, while the mixer is on high speed. At first, your egg whites will deflate and look like the butter is causing them to split, but don’t panic! It will come back together. It takes a couple of minutes for the butter and meringue to become best friends.
Once you have added all the butter, add the vanilla extract and mix first on low speed to combine, then on high speed for 5 minutes. You’ll know it’s done when the frosting has come together, has turned pale in color (if you’re making vanilla) and is nice and fluffy.
To finish, add the white chocolate and mix until well combined.