Roughly chop the chocolate and add it to a small glass or metal mixing bowl. Select a pot that the bowl nests into. You want the bowl to hover over the water, not touch it. Heat the pot over medium-low heat to melt the chocolate. Once the chocolate is melted, remove it from the heat to cool while you prepare the cookie dough.
In the bowl of a stand mixer fixed with the paddle attachment, cream the butter until light and fluffy. Add the coconut sugar and beat until combined. Beat in the egg followed by the vanilla. Stop the mixer and scrape down the sides of the bowl as needed.
In a medium mixing bowl, sift together the almond flour, tapioca starch, cocoa powder, and baking soda. Whisk well to combine the ingredients.
Pour the dry ingredients into the bowl of the mixer along with the wet ingredients. Mix on medium-low until just combined.
Using a baking spatula, fold the melted chocolate into the cookie dough until fully incorporated. Fold in the apple cider vinegar.
Transfer the cookie dough to the fridge to chill for 45 minutes.
Preheat the oven to 350°F and line a half sheet pan with parchment paper. Using a #24 cookie scoop (3 tablespoon size), portion the dough into 16 balls. Roll each ball of dough between your palms until they are nicely rounded.
Place the balls of cookie dough on the lined half sheet, leaving two inches between each cookie. I can usually fit eight cookies on a half sheet. Bake for 12 minutes, rotating the pan halfway through cooking. The cookies are ready once they have flattened and have a cracked surface. I like to put the remaining balls of cookie dough back in the fridge to stay cool while the first batch bakes.
Remove the cookies from the oven and allow them to cool on the pan for five minutes. This step is crucial! Sprinkle a generous amount of flaky salt over the cookies. After five minutes, transfer the cookies to a cooling rack to cool completely. Repeat with the remaining cookie dough.