My personal favorite (and the filling used in my photos here) is my white chocolate buttercream; it's surprisingly not too sweet and pairs nicely with my French macaron recipe. Other good choices include my favorite chocolate frosting, chocolate ganache (let it get firm before using), Swiss meringue buttercream, or save your yolks and make lemon frosting. I also have a raspberry frosting coming soon that will be a wonderful filling! I personally find classic American buttercream frosting to be too sweet for this recipe.