Place egg whites and sugar in heat proof bowl (not metal).
Set bowl over pan of gently simmering water, and whisk until the sugar has dissolved and the egg whites are hot to the touch, about 5 minutes. Test by rubbing the mixture between your fingers; it should feel completely smooth.
Transfer egg and sugar mixture to the bowl of a stand mixer fitted with the whisk attachment.
Beat on high speed until mixture has cooled completely and formed stiff and glossy peaks, about 8-10 minutes.
Reduce speed to low. Add the butter, one piece at a time, and beat until incorporated after each addition. Don’t worry if the buttercream appears curdled after all the butter has been added; it will become smooth again with continued beating, so be patient!!
Add maple syrup, vanilla and cinnamon and beat just until combined.
Switch to the paddle attachment and beat on low to eliminate any air pockets, about 5 minutes.
**If you are using the buttercream within a few hours, cover the bowl with plastic wrap, and leave at room temperature in a cool environment.
***If not, then store in the refrigerator, up to 3 days. Before using, bring the buttercream to room temperature, and beat on the lowest speed with the paddle attachment until smooth, about 5 minutes.