Preheat the oven to 300F and line a baking sheet with parchment paper. To prepare the cookies, combine all of the cookie ingredients in a medium mixing bowl and stir well to create a sticky, uniform dough. Use a heaping teaspoon to scoop the dough into 12 mounds onto the lined baking sheet, then use your fingers to shape the dough into 2-inch circles about 1/4-inch thick. (You can get your hands wet to help prevent the dough from sticking to your hands.) Press your thumb into the center of each circle to create a well for the peanut butter. Bake the cookies at 300F until the edges are lightly golden, about 18 minutes, then set aside to cool completely while you prepare the peanut butter layer.
To prepare the peanut butter filling, simply stir together the peanut butter, maple syrup, and salt until smooth. Drop a teaspoon of the mixture onto each cooled cookie, and then use your fingers or a spatula to spread the peanut butter over the top of the cookie. Place the peanut butter topped cookies in the freezer to set while you prepare the chocolate topping.
To prepare the dark chocolate coating, simply melt together the dark chocolate chips and coconut oil or butter in a double boiler. (I use an oven-safe bowl that fits over a small sauce pot filled with about an inch of boiling water.) Once the chocolate is melted and smooth, remove the cookies from the freezer and spoon the melted chocolate over the top of each cookie, making sure to coat the sides. Once all of the cookies are coated, place them back in the freezer to set until the chocolate is firm. Serve immediately, or keep them chilled in the fridge until ready to serve. (The chocolate may melt at room temperature if your room is too warm. Since it's currently winter, my kitchen has been cool enough lately to store them at room temperature without them melting, and I do prefer the texture of the shortbread cookie at room temperature when it's possible.)
*This recipe is vegan if you use coconut oil, however the coconut oil does impart a slight coconut flavor to the final cookie. If you don't need a vegan recipe, real butter also works well without adding the coconut flavor.Also, if you'd like to coat the bottoms of the cookies in chocolate, simply remove them from the freezer once the chocolate tops have set, flip them over, and spoon more chocolate over the bottom. Place in the freezer to set the bottoms, and then serve. My husband and I both preferred the batch of cookies I made without the chocolate bottoms, so I recommend skipping that extra work!