Place racks on upper and lower thirds of oven and preheat to 325F. Spread out coconut on rimmed baking sheet in an even layer and toast on top rack, shaking sheet occasionally, until golden brown (8-10 minutes). Transfer coconut to a plate or small bowl
Whisk egg whites, brown sugar, and kosher salt in a large bowl vigorously until frothy and pale. Whisk in vanilla, then fold in toasted coconut
Scoop out heaping tablespoonfuls of coconut mixture, pressing 1-2 pieces of chocolate inside. Add pinches of coconut mixture as needed to completely encase chocolate. Divide macaroons between two parchment-lined baking sheets, placing about 2“ apart. Sprinkle each with flaky sea salt
Bake, rotating baking sheets top to bottom and back to front halfway through, until golden brown, about 15-17 minutes. Let cool on baking sheet 10 minutes, then transfer to a wire rack and let cool completely