Check the ganache to see it has thickened. It shouldn't be runny, but should be thick enough to pipe, like the consistency of very soft butter. If not, place it in the microwave for no more than 5 seconds. Stir well. Place #18 open star pastry tip
in a pastry bag and fill with ganache. Alternatively, fill a plastic zip bag with ganache, forcing it into one corner and cut a small hole in the corner. Pipe the ganache just inside the edge of a cookie in a circular motion moving from the edge to the center. Sprinkle a few flakes of Fleur du Sel over the ganache and top it with another cookie.