Combine spices with chai simple syrup and set aside.
Wipe down your mixing bowl, whisk and paddle attachments with a small amount of white vinegar.
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 your fingertips. The mixture will be hot.
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 gradually add in your chai simple syrup spice mix and vanilla extract.
Turn your mixer up to medium to high speed and mix for about 4 minutes until buttercream is completely incorporated, silky, and smooth. Turn the mixer to low speed and mix for 2-3 more minutes. This helps to remove any air bubbles that have formed.
Remove bowl from mixer and use spatula to remove any remaining air bubbles. Push the spatula against the side of the bowl, squishing the buttercream into the bowl.