In the bowl of a stand mixer with the whisk attachment, beat the egg whites on medium-low for 1 minutes, or until they foam.

Add the cream of tartar. Beat and eventually raise speed to high.  When the egg whites become thicker, gradually add the sugar. Beat for 5 to 10 minutes until a thick meringue forms.

Turn the mixer to low speed and add the butter one tablespoon at a time, making sure that the butter is fully incorporated before adding more. At this point, the meringue will deflate and look curdled- this is normal. Keep mixing. Once the butter has all been added, mix until the buttercream is smooth and silky. If it looks curdled, keep mixing, it will come together.

Whisk in the vanilla extract and salted caramel to taste.