Bring two inches of water to a simmer over medium-high heat. Once it's simmering, reduce the heat to medium-low.
Place your egg whites and brown sugar together in the metal bowl of your stand mixer and whisk to combine.
Place the bowl on top of the simmering water and whisk continuously until the grains of sugar are dissolved. Don't walk away or you could accidentally cook your eggs.
Once you cannot feel any grains of sugar, take the egg mixture off the heat and attach it to your stand mixer with the whisk attachment. Turn the mixer onto high and let your eggs whip until you reach stiff STIFF peaks. Stiff peaks is when the meringue stands straight up from the whisk. This can take 5-15 minutes depending on the strength of your mixer.
Because I'm impatient, I like to scoop out my meringue and place it onto a cookie sheet to cool down quickly in the fridge. You can stick the whole bowl in the fridge but it takes much longer. We can't whip in the butter until the meringue is cool or it will melt the butter and we'll get brown sugar soup!
After about 10 minutes, your meringue shouldn't feel warm anymore, scoop it back into the bowl and start mixing on medium speed while adding in your chunks of room temperature butter.
Add in your salt, vanilla, and cinnamon.
Increase the speed to high and whip the buttercream until it's light and fluffy and doesn't taste like butter anymore.