Preheat the oven to 350° F. Line baking sheets with parchment paper or silicone baking mats. In a medium bowl, sift together the flour, cocoa powder, baking powder, and salt. In the bowl of an electric mixer, beat the butter, brown sugar, and granulated sugar on medium-high speed until well combined and smooth. Add the egg, egg yolk, and vanilla and beat until combined. On low speed, gradually add the flour mixture. Fold in the chocolate chips by hand. Using a medium spring-loaded cookie scoop, drop the dough by 1½ tablespoon rounds onto the prepared baking sheets. Bake for about 10 minutes or until the cookies are
set. Let them cool on baking sheets for 10 minutes before removing the cookies to wire racks to cool completely.
For the topping, combine the milk, sugar, egg yolk, and butter in a small saucepan. Cook over medium heat, stirring frequently,
until the mixture has thickened, about 10 minutes. Remove from the stove and stir in the vanilla, coconut, and pecans. Let cool until thickened. Spread on top of the cooled cookies. Drizzle the melted chocolate over the cookies. Let everything set for at least 15 minutes before serving. The cookies are best when served the day they’re made.