Preheat the oven to 350°F. Line two baking sheets with parchment or spray with cooking spray.
In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt. Set aside.
In a large bowl or the bowl of a stand mixer, beat together the butter, brown sugar and sugar. Add the egg, egg yolk and vanilla and mix to combine. Then add the flour mixture and mix to combine. Gently fold in the chocolate chips.
Scoop the dough into 1-inch balls and place on prepared baking sheets. Bake for 10 minutes, then remove from oven and let cool on baking sheet before removing to a cooling rack.
Coconut Pecan Frosting:
While the cookies are cooling, combine the evaporated milk, sugar, egg yolk and butter in a small saucepan. Bring the mixture to a simmer over medium-heat, stirring frequently. Simmer for 10 minutes, or until the mixture has thickened and turned a caramel color.
Remove from heat and stir in coconut and pecans. Allow the frosting to cool slightly and thicken, then spread on cooled cookies. Drizzle with semisweet chocolate and let cool before serving.