Preheat the oven to 325ºF and line two baking sheets with silicone mats. You can use parchment instead but keep a close eye on your cookies as the bottoms may brown quickly.
In a large bowl, beat the butter with the sweetener until light and fluffy, about 2 to 3 minutes. Beat in the egg and vanilla until well combined.
Add the almond flour and salt and beat until the mixture is uniform, scraping down the sides of the bowl and the beaters as necessary.
Cut a 1 inch diameter hole in a piping bag and fill with the dough. Pipe 2 inch long logs onto the prepared baking sheets, leaving at least 2 inches between each cookie.
Bake 15 to 20 minutes, until the edges are golden brown. Turn the oven off and let the cookies sit inside, keeping a close eye on them, for another 10 to 15 minutes to help them crisp up.
Let cool completely before filling.
In a small microwave-safe bowl, combine the chocolate and the butter. Melt on high in 30 second increments, stirring in between. Alternatively, you melt double-boiler style on the stove.
Spread the bottom of one cookie with about 1 teaspoon of the chocolate filling. Top with another cookie. Place on a tray and refrigerate 10 minutes to set the filling.
