• Preheat your oven to 325°F. Line a large rimmed baking sheet with unbleached parchment paper and set it aside.
• In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, place the butter and beat until creamy (about 2 minutes).
• Add the egg yolk, confectioners’ sugar, vanilla and salt and beat to combine well (1 minute more).
• Add the flour and xanthan gum, and mix until fully combined. The cookie dough should be thick but not stiff.
• Scoop the dough into portions about 1 tablespoon each with a 2-inch cookie scoop (a small #60 cookie scoop works best) or 2 spoons. Place in mounds about 1 1/2-inches apart on a prepared baking sheet.
• Roll each mound of dough lightly into a ball, without packing the dough, then roll each ball in the granulated sugar to coat on all sides, and replace on the baking sheet.
• Using an index finger, small dowel or the back end of a wooden spoon, poke a deep hole in the center of each ball about 1/2-inch wide and 3/4-inch deep.
• If you plan to fill the cookies with chocolate ganache, leave the holes empty.
• Do not chill the cookie dough.

To fill with jam

• Warm the jam slightly in the microwave to make it thickly pourable. Using a small spouted measuring cup or a small spoon, fill each of the holes with the jam.
• Place the baking sheet in the center of the preheated oven and bake until the cookies are set and just beginning to brown around the edges, 10 to 12 minutes.
• Allow the cookies to cool on the baking sheet, undisturbed, for at least 10 minutes or until firm.

To fill with chocolate ganache

• If your cookies are hollow, check for any holes that have begun to close up, and press them down again gently with your finger or the same dowel you used to create the hole.
• Place the chopped chocolate in a small heat-safe bowl. Heat the heavy whipping cream in a small, heavy-bottom saucepan or a microwave safe bowl just until it begins to simmer.
• Pour the hot cream over the chopped chocolate and allow the mixture to sit for about 3 minutes, or until the chocolate begins to melt. Stir until the chocolate is melted and the mixture is smooth.
• Using a spouted measuring cup or a small spoon, pour the ganache into the open well of each cookie. Allow the filling to set at room temperature or in the refrigerator and serve.