Set up a double broiler or a saucepan with an inch or two of water, making sure the water is at least 1-2 inches below the bottom of the bowl you will be placing on the saucepan.
Mix all ingredients, minus the vanilla extract in a separate bowl.
Pour the mixed ingredients into the bowl of a standing mixer or in a large heat safe bowl, being careful not to splash the mix along the sides of the bowl.
Place the bowl over the prepared saucepan.
Start to simmer the water and heat the mixture until it reaches 165 ℉ and all the sugar has dissolved, about 5-10 minutes from when the water starts to simmer.
While heating, gently mix to ensure even cooking. If any sugar egg mixture has splashed on the sides you can use a pastry brush dipped in water to wash it down or use a spatula to scrape down the sides of the bowl. Any undissolved sugar can cause the frosting to get grainy so it’s important all the sugar crystals have dissolved.
The egg white mixture will be fairly warm to the touch, but you can check to see if the sugar has dissolved by rubbing a bit between your fingers. If it is still grainy, heat it longer until you can no longer feel any grains of sugar when you test it.
Once all the sugar has dissolved and the mix is 165 ℉, remove from heat and place the bowl in a standing mixture and mix on medium high for 5-7 minutes until the egg whites whip up and the mixture starts to cool. Once it starts to whip up, add the vanilla. You can also use a hand mixer to mix it.
This frosting is best used the day you make it but it will last up to a day in the fridge.