In the top of a double boiler, combine egg whites, sugar, vanilla extract, and cream of tartar. Cook over simmering water, whisking constantly, until sugar dissolves and egg whites are warm to the touch, approximately 2 minutes.
Transfer warm egg white mixture to a large bowl, and beat with a mixer
fitted with the whisk attachment at high speed until mixture is glossy and very stiff peaks form, approximately 5 minutes. Use immediately.
Recipe by TeaTime Magazine at https://teatimemagazine.com/key-lime-shortbread-tartlets-recipe/