Combine the butter and confectioners' sugar in a stand mixer fitted with the paddle attachment and beat on medium speed until light and creamy, about 2 minutes. Add the egg, vanilla extract and salt and mix on low speed until combined. Add the flour cup by cup, mixing on low speed after each addition, just until incorporated. (The dough will look crumbly.) Scrape down the sides of the bowl and dump out onto a lightly floured surface. Use your hands to push the dough into a mound. Divide the dough in half and flatten each half into a thick square. Wrap both pieces of dough in plastic and refrigerate until firm, at least 3 hours or up to overnight.
Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper and rest the dough at room temperature for 15 minutes to soften slightly.
Sprinkle flour over the counter and a rolling pin. Remove the dough from the plastic wrap and dust lightly with flour. Roll each piece of dough into a square that is 1/4-inch thick. Cut the dough into 4-by-2-inch rectangles (or use a cookie cutter). Carefully place on the prepared baking sheets. Continue until all of the dough has been cut into rectangular cookies.
Bake the cookies until golden brown on the edges, 8 to 10 minutes. Cool for 2 minutes on the baking sheets, then transfer to a wire rack to cool completely.
Meanwhile, prepare the edible chalk. Heat the candy melts in a bowl in the microwave until melted, about 1 1/2 minutes. Pour into a pastry bag and cut the tip. Insert the tip of the pastry bag into 1 end of a silicone tube mold. Apply even pressure to the bag to fill the mold with candy, then tap the mold on the counter to flatten out the candy. Refrigerate until firm, about 15 minutes. Remove from the molds and set aside for cookie decoration.
Prepare the royal icing: You will create 2 batches of black icing, one that's stiff for piping the cookie borders, and one that's loose to flood the cookie surface. Place 1/2 cup royal icing in a bowl. Pour in 10 drops of black gel food coloring and whisk together until the color is evenly incorporated. Pour into a squeeze bottle. Following the shape of the cookie edges, pipe a border of stiff icing on each cookie.
Place the remaining royal icing in a bowl. Pour in 15 drops of black gel food coloring and enough water to reach the consistency of pure maple syrup (add water by the teaspoon). Whisk until the color is evenly incorporated and the desired consistency is reached. Pour into a squeeze bottle. Flood the cookie surface with this icing, filling the rectangle evenly with color. Use a toothpick to push the icing into the corners of the rectangle or to poke any air bubbles that may form. Repeat until all the cookies have been decorated. Allow the cookies to dry for at least 4 hours.
When ready to serve, present the chalkboard cookies with the edible chalk.