Wipe down the bowl of your electric stand mixer, whisk and paddle attachments with a small amount of white vinegar. Having any fat residue on these surfaces can impact how your meringue whips up.
Create a double boiler on your stove by filling a medium saucepan with a couple inches of water. Do not let the water touch the base of the bowl. Put your egg whites and sugar in your bowl and place on top of the saucepan. Give them a whisk to combine.
Cook your egg white/sugar mixture over medium heat stirring constantly until the mixture reaches 160 degrees F on a candy thermometer or until you can no longer feel any sugar granules between two fingertips. The mixture will be warm to touch but not scalding.
Transfer your egg white/sugar mixture to the bowl of your electric stand mixer with the whisk attachment in place and whisk on high until the outside of your mixing bowl is no longer hot and you’ve achieved stiff peaks in your meringue. This can take between 8-12 minutes.
Switch to the paddle attachment. Turn your mixer to low speed. Add in your cubed butter (2 tablespoon in size) one at a time, allowing each to be incorporated before the next addition.
Scrape down the sides of your bowl, turn mixer to low speed and add in your vanilla and rose water. Begin by adding 1 teaspoon of rose water and mix to combine. Add more as desired. Be cautious not to add too much rose water because it can lead to the buttercream tasting very artificial.
Turn your mixer up to medium to high speed and mix for about 2-3 minutes until buttercream is completely incorporated, silky, and smooth.