Combine the shredded coconut, granulated sugar, potato starch, and salt in a large mixing bowl.
Blend the dry ingredients thoroughly with a fork, pressing apart any visible clumps of potato starch.
In a separate small bowl, whisk the egg whites with the vanilla extract until the mixture becomes frothy.
Pour the egg-white mixture over the coconut ingredients. Stir with a fork until every portion of coconut is evenly moistened.
Allow the mixture to stand for 20–30 minutes so the shredded coconut can absorb the liquid and soften.
Preheat the oven to 325°F (163°C). Line a baking sheet with parchment paper or a silicone baking mat.
Stir the rested coconut mixture once more with a fork.
Scoop tablespoon-sized portions onto the prepared baking sheet, arranging the mounds relatively close together.
Because the mixture is delicate, dampen your fingers and carefully form each portion into a loose dome or haystack shape. The macaroons do not need to appear perfectly uniform.
Bake for 20–25 minutes, or until the lower edges are golden and the exposed coconut tips begin to brown.
Remove the baking sheet from the oven as soon as the macaroons are properly colored to prevent them from becoming dry.
Leave the baked macaroons undisturbed on the baking sheet until they have cooled completely.
Do not attempt to move them while warm, as their delicate structure may cause them to crumble. Once fully cooled, they will be considerably easier to handle.
Arrange the cooled macaroons on a flat surface lined with parchment paper.
Place 9 ounces of the dark chocolate in a microwave-safe bowl. Heat at 50 percent power for 1 minute, then stir thoroughly.
Continue microwaving the chocolate in 15-second intervals, stirring after each interval, until it is completely melted and smooth.
Dip the bottom of each macaroon into the melted chocolate, allowing the coating to extend approximately 1/4 inch up the sides.
Hold each macaroon over the bowl briefly so the surplus chocolate can fall away. A fork may be used to guide off excess chocolate and create a cleaner finish.
Return the dipped macaroons to the parchment-lined surface and repeat until every macaroon has been coated.
Melt the remaining 4 ounces of dark chocolate until smooth.
Spoon the melted chocolate into a small resealable plastic bag. Press the chocolate toward one corner, seal the bag, and carefully trim a very small opening from the tip of that corner.
Pipe the chocolate over the macaroons in a zigzag pattern.
Leave the macaroons undisturbed until the chocolate has hardened completely. To shorten the setting period, place the tray in the refrigerator.