Add the sugar and the egg whites into a heatproof mixing bowl.
Whisk constantly until the mixture is hot, sugar is dissolved and the egg whites are a little frothy. About 5 minutes. It should look like shiny marshmallow cream. This step is very important and you can use a thermometer to check the temperature. It should get to 160 degrees.
With the whisk attachment, transfer the egg mixture to a mixing bowl and beat on medium speed until cooled and thicker for about 5 minutes.
Add in one stick of room temperature butter at a time, beating until smooth.
On low speed, add in the extract.
If the buttercream curdles or separates, just keep beating on high until it comes back together. You should end up with a shiny, smooth, and absolutely delicious buttercream frosting!
Notes
To change the flavor add in different extracts! Use a candy thermometer to check the temperature of the egg and sugar mixture.