On a stovetop, mix together the water and sugar, cover with a lid and bring to a boil on medium-high heat.
Keep the lid on the pot for 3-4 minutes and bring to ensure all the sugar granules are dissolved, otherwise, your sugar can get gritty and crystalize.
Remove the lid, insert the candy thermometer carefully and continue cooking on medium-high until the syrup reaches 240° F.
When the sugar solution is at about 235° F, begin whipping the egg whites on high speed. Add the salt to the egg whites.
When the egg whites reach soft peaks, pour the sugar solution in a steady stream on to the whipping whites while mixing on low speed.
Continue whipping the egg/sugar mixture until it reaches stiff peaks. I wrapped an apron around my bowl with an ice pack to help the meringue cool down faster. You can also cool the meringue by scooping it out of the bowl and placing it into the fridge for 15 minutes.
Once the meringue is cooled, whip in soft butter and vanilla until the buttercream is light and fluffy and no longer has a butter taste.