Preheat oven to 350 degrees F. Spread coconut into thin layer on a baking sheet. Toast in preheated oven for 15 minutes, stirring every few minutes to ensure it browns evenly and the coconut does not burn. Allow to cool completely.
In a food processor, process toasted coconut, sugar, and salt until it is finely ground, about 30 seconds. Add the butter, egg yolk, and vanilla and process another 20-30 seconds until smooth and creamy. Scrape the sides, add the flour, and pulse 5-10 times until a ball forms. Do not over mix once the flour has been added.
Lay out a long piece of plastic wrap. Shape the dough into a rectangular log so that its about 2 inches wide and 1 inch tall. Once I get the general shape, I wrap tightly with plastic wrap and then I use two long boxes (one foil box and one plastic wrap box) to press down on the top and push the sides in to make a nice uniform rectangle shape. Slide that rectangle dough log onto your plastic wrap box for easy transport and slide it into the refrigerator where it must chill at least 2 hours.
Preheat oven to 350 degrees F. Prepare baking sheets with parchment paper or a silpat.
Slice cold dough into ¼ inch thick slices and arrange on the baking sheets. They won't spread much if the dough is cold so you can space them about an inch apart. Bake until just slightly golden brown on the bottom, 10-12 minutes. Then allow cookies to remain on baking sheet another 10 minutes before you transfer them to a cooling rack to fully cool. Repeat until all cookies have been cooked.
For the chocolate drizzle, melt in pot over very low heat or in the microwave on 50% power. When fully melted, you can dip a rubber spatula in the chocolate and then wave it back and forth over the cookies to get a nice thin line. Continue dipping and waving until all the cookies have the right amount of chocolate - only you can be the judge of that. They look best if you don't change direction with the waving (I speak from experience). This step will make a mess so set your cookies in the center of your counter top, not on your carpet. :)
Once chocolate hardens, store in air tight container.