Prep and mix the cookie dough. Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside. In the bowl of a stand mixer, cream together the softened butter, brown sugar, and sugar until the mixture is light and fluffy. Add the egg and vanilla, mixing until fully combined.
Add dry ingredients. Add the flour, cocoa powder, baking soda, and salt. Mix well on low speed, scraping down the sides as needed. The dough will be thick and rich.
Shape and bake. Use a large cookie scoop or ice cream scoop to divide the dough into equal portions. Roll each portion into a ball and gently flatten to about ½ inch thickness with the bottom of a glass or your palm. Place on the prepared baking sheets and bake for 10–11 minutes or until the centers are just set. Let cool completely on the pan.
Toast coconut and pecans. In a dry skillet, toast coconut flakes and chopped pecans over medium heat for 3–5 minutes until golden and fragrant. Stir frequently and watch closely to prevent burning. Remove from heat and set aside.
Make the frosting. In a medium saucepan, combine brown sugar, milk, butter, egg yolk, vanilla, and salt. Stir and cook over medium-low heat until melted and just starting to bubble. Remove from heat and stir in the toasted coconut and pecans. Let cool slightly to thicken.
Assemble and drizzle. Spoon a layer of the coconut pecan frosting onto each cooled cookie, spreading to the edges. Melt the chocolate chips in the microwave or a double boiler, stir until smooth, and drizzle over the frosted cookies. Let them set before serving or storing.