In a small bowl, whisk together the flour, cocoa powder and salt. Set aside.
In the bowl of a mini stand mixer (or small bowl using an electric hand mixer), cream the butter and sugar together until fluffy, about 1 minute. Add the egg yolk and vanilla, and beat until combined.
With the mixer running, slowly add in the flour mixture, and beat until a sandy-looking dough forms.
Gather the dough out of the bowl, roll into a log about 4-5” long. Wrap the log in parchment paper and chill in the fridge until the oven is preheated and ready to go (about 20 minutes).
Preheat the oven to 350°F and line a baking sheet with parchment paper.
When the oven is fully heated, remove the dough from the fridge and slice into 1/4” slices.
Place the cookie slices on the prepared baking sheet at least 2-inches from each other, and bake for 11-12 minutes, until the top of the cookie appears dry. The cookies won’t spread much, and a few might start to crack.
Let the cookies cool completely on the sheet before moving to a wire rack.
In a small bowl, melt the chocolate and coconut oil in 30-second pulses in the microwave. Stir between each pulse, and stop heating before all of the chocolate chunks have melted. Set aside to rest while the last bit of chocolate melts.
Dunk each cookie half-way into the chocolate, and place on a parchment sheet to set. Sprinkle with flaky sea salt before serving, if desired.