In a bowl, use a whisk to beat the egg yolks with the maple sugar until the mixture is creamy and pale yellow. Add the flours, baking powder, and salt, mixing with a wooden spoon. Incorporate the butter.
Lightly knead the dough until it’s nicely smooth, wrap in plastic, and refrigerate for at least 30 minutes.
Preheat oven to 180° C (350° F).
Lay the dough in a 20 cm (8 in.) long by 1 cm (3/8 in.) high rectangular springform pan. Bake for about 30 minutes or until the shortbread is browned.
Once out of oven, cut into 10 cm by 1 cm (4 in. by 3/8 in.) rectangles.
In a medium saucepan, heat the syrup, cream, butter, and maple sugar over a medium stove.
In a mixing bowl, beat the eggs and egg yolks, then add salt. Add to the syrup mixture, whisking vigorously to incorporate. Set to a boil.
Pour sauce into a strainer lined with cheesecloth and positioned over a bowl. Lay plastic wrap directly on the sauce and refrigerate until ready to use.