In a medium bowl, whisk together the melted butter, granulated sugar, brown sugar, egg yolk, and vanilla extract. Mix in the flour, cocoa powder, baking powder, and salt until everything is combined. Fold in the chocolate chips and sliced almonds. Chill the dough in the refrigerator for 30 minutes.
While the dough is chilling, preheat your oven to 350°F. Line a baking sheet with parchment paper or a silicone baking mat.
Form dough into one giant ball and place in the center of the prepared baking sheet. Top the cookie dough with a few more chocolate chips and almonds, if desired. Pop into the oven and bake for 19-22 minutes. After 17 minutes or so, pull the cookie out of the oven and press the mini marshmallows into the top of the cookie. Pop the cookie back into the oven and continue baking for another 2-5 minutes.
Once done baking, use a spatula or a spoon to gently pull in the edges of the cookie if it spread. Allow the cookie to cool for 5-10 minutes before transferring to a cooling rack. Enjoy!