Preheat the oven to 350°F. Line a baking sheet with parchment paper.
In a large mixing bowl, add the almond paste, sugar, salt, and ground almonds. Mix it with your hands, breaking up the almond paste by squeezing it between your fingers until the mixture forms fine crumbs. (You can also use an electric hand whisk for this, but the almond paste I get is usually too dry for the mixer to break it up easily.)
Add the egg whites and almond extract to the mixing bowl, and mix with a wooden spoon or an electric hand whisk until it forms a smooth, wet, sticky dough.
Take a few tablespoons of the dough at a time and roll it between your hands or on a clean work surface into a log about 4 inches long. (Because the dough is so sticky, it helps to wet your hands with cold water when shaping and handling the dough. This prevents the dough from sticking to your hands as you roll and shape it.)
Place the sliced almonds for the topping in a large shallow bowl. Roll each dough log in the sliced almonds to coat them.
Place the almond-coated dough log on the prepared baking sheet, and bring the two edges closer together to form a horn or horseshoe shape. Place the cookies at least 1 inch apart on your baking sheet, since they'll spread a little bit in the oven.
Bake the cookies for 12 to 15 minutes, until the cookies are dry on top and golden on the bottom. The cookies will still be soft when they’re done baking, so gently transfer them to a cooling rack with a spatula to cool and become firm, for about 20 minutes.
Melt the chocolate in a microwave-safe bowl on 30 second intervals, stirring after each time. Dip the ends the cookies in the melted chocolate, then return the cookies to the parchment paper-lined baking sheet to allow the chocolate to cool and harden, for at least 1 hour. Enjoy!!
Notes
If your almond paste is really hard and/or dry, you can shred it with a cheese grater or pulse it into fine crumbs in a food processor before mixing it with the other ingredients.
Don't swap in marzipan for the almond paste, since it could make your cookies too soft and way too sweet.
Store your cooled cookies at room temperature in an airtight container for up to 1 week. I don't recommend storing them in the fridge, since they could absorb moisture and lose their crispy exterior texture.