I wish I had my book with me, but I don’t have a computer at home…and I don’t have the book in the office…but here’s what you do.
Go to Neoclassic buttercream recipe.
There is a variation for Maple
Follow the amount of syrup, sugar, and extract for that recipe, but instead of egg yolks, use 5 egg whites with 1/4 cup sugar beaten into them (like the mousseline recipe) and heat the syrup to 248 degrees (like the mousseline recipe). (I don’t remove any sugar from the syrup to compensate for the 1/4 cup added to the whites since I like things a little on the sweeter side).
I find it works fine.
Let me know