Step 1Heat the oven to 350 degrees. In a large bowl, cream the butter and sugar until light, fluffy and thoroughly combined, about 4 to 6 minutes.
Step 2Add the vanilla, egg yolk, cinnamon, salt and nutmeg and mix to combine, scraping down the sides if necessary. Turn the mixer to low and mix in the flour until thoroughly incorporated.
Step 3Press the dough into a parchment-lined and sprayed 16-by-11-inch baking sheet. Bake until golden brown, about 17 to 20 minutes. Cool for 3 to 5 minutes. Using a 2-inch round cookie cutter, cut out cookie bases. Place the cookies on a wire rack and allow to cool completely.
Step 4Remove the pan of marshmallow from the refrigerator and, using the same cookie cutter, cut out rounds. Place each marshmallow round onto a cooled cookie.
Step 5Place a large sheet of parchment on a work surface and put a wire rack on top of it. Place the cookie-marshmallow bases onto the rack, spacing them 2 inches apart. If you need to do this in 2 batches, do so.
Step 6Place the chocolate in a large bowl. Heat the cream just until it comes to a boil and pour it over the chocolate. Whisk the mixture until it is completely smooth.
Step 7Transfer some of the ganache to a glass measuring cup with a spout, or to a small pitcher. Carefully pour the warm ganache over the bases, taking time to coat each piece evenly. Refill the pitcher as necessary, and reuse the glaze that has dripped onto the parchment. If the ganache begins to cool and harden, rewarm it, but do not overheat it. Continue coating the marshmallow bases until you have done them all.
Step 8Using a metal spatula, move the coated cookies to another wire rack. This will prevent the ganache from pooling around the bases of the cookies. Allow the cookies to cool completely. Refrigerate until the chocolate is completely set, at least 2 hours, before decorating (if desired) or serving.