This is an Italian buttercream made with maple syrup instead of sugar.
Bring the maple syrup to a boil in a saucepan over medium-high heat (use a medium size pan as the syrup will boil). Clip a thermometer to the pan and cook until the mixture reaches 238-240°F.
While the maple syrup is cooking, whisk the egg whites and a pinch of salt on medium speed in the bowl of a stand mixer until they form medium firm peaks, about 2-3 minutes. Once the maple syrup reaches 238°F, remove from the heat and gradually and steadily pour it down the side of the bowl, resting the edge of the pan on the mixer bowl so that the syrup does not hit the whisk attachment and splatter.
Whisk the meringue until stiff peaks form and the bowl cools down, about 8 minutes. Swap out the whisk attachment for the beater. Beat on medium-high speed, adding the salt and then the butter piece by piece until incorporated and fluffy (Note: start with 1/4 tsp. sea salt to start, adding more to taste). When the beater begins making a slapping sound the buttercream is done. The buttercream can be kept covered and refrigerated for up to 2 weeks.
