Place sugar and egg whites in a large heat proof bowl.

Whisk continuously over a double boiler (saucepan of simmering water, your bowl should not touch the water!) Make sure you whisk continuously, I would suggest a hand mixer. But if you are like me, and like to work (hard) for your dessert, hand whisk it.

Whisk and whisk until the mixture comes to a temperature of 160F or 70C.

Remove from heat and move mixture to stand mixer.

Whisk until mixture cools down, it can take about 10 minutes. This is when I took my butter out from the fridge and cut them to about 1 inch by 1 inch cubes.

When meringue is cooled, glossy, and formed firm peaks.

Change to paddle attachment, add in butter cube, one by one on medium speed, allowing butter to be combined before adding another.

Then add your salt and vanilla extract, beat until combined.

Frost your cuppies! :)