The classic filling for French Macaron Cookies is this recipe, Italian Buttercream, of various flavors. This is not the typical Italian Meringue or Mousseline Buttercream or IMBC that uses a sugar syrup.
FROSTING, ICING, ETC RECIPE HELP

INGREDIENTS
2 large egg whites
1/3 cup plus 1 tablespoon sugar
1/2 cup (4 ounces) unsalted butter at room temperature, cut into slices
1/4 teaspoon any flavor extract or a few drops any flavor candy oil

INSTRUCTIONS
1. In the bowl of a stand mixer, whisk together the egg whites and sugar.

2. Set the bowl over a pot of simmering water and heat the mixture, whisking often, for 3 to 5 minutes, or until it feels warm and sugar has dissolved.

3. Transfer the bowl to the stand mixer fitted with a whisk attachment. Whip the warm egg mixture on high speed until stiff and shiny, taking about 3 to 5 minutes.

4. Add the butter, one slice at a time, and continue to mix until all the butter is thoroughly incorporated. Add any flavorings and refrigerate for 1 hour or until it becomes firm.

STORAGE
The buttercream can be kept, covered and refrigerated for up to 1 week.