Add eggs whites + sugar to a heat safe bowl and mix to combine.
With the stove on medium high heat, bring the water to a simmer. Place egg white mixture over the simmering water, whisking constantly until it reaches 160 degrees F or until you can longer feel the sugar between your fingers.
Pour hot egg mixture into your mixing bowl or kitchen aid and beat on high with whisk attachment until you form a meringue with stiff peaks.
While that is beating, cut your 2 cups of softened, room temp butter into tablespoon pieces. Once your meringue is set, switch to your paddle attachment and slowly add butter, 1 - 3 tbsp at a time. Mix well between each addition, continue until all butter is gone.
Once all of your butter is added, switch back to your whisk attachment and beat on high until mixture is light and fluffy.
If your mixture curdles, you can heat the side of the bowl slightly with a torch or over a gas stovetop for about 5 - 10 seconds. Then continue beating on high until you have a light, white, fluffy buttercream. At this point, you can add in any flavoring or colouring if you choose to include it!