Melt the semi-sweet chocolate in the microwave and let cool.
In a mixing bowl, whisk the flour, cocoa powder, baking powder, and salt together. Set aside.
In the bowl of a stand mixer, with the paddle attachment, beat the butter, brown sugar, and granulated sugar together for 2 minutes. Add the eggs and vanilla and mix until smooth. Mix in the melted chocolate.
Add the flour ingredients and mix until combined, scraping the sides as needed.
Use a wooden spoon or a spatula to mix in the walnuts, ½ cup marshmallows, and ½ cup of the chopped milk chocolate bars.
Cover dough and refrigerate at least 2 hours. If chilled longer, let it sit on the counter for 30 minutes before rolling.
Preheat theoven to 350 degrees Fahrenheit and line baking sheet with parchment paper.
Scoop the dough and form into 1 ½ inch balls, about 24. Place the dough 3 inches apart on the prepared pan.
Bake for 11 minutes and then press 3 marshmallows into the tops of each cookie. Return to oven and cook for 1 more minute. Press in a couple more chocolate chunks and let cool on the pan for 5 minutes. Transfer the cookies to a cooling rack to cool before enjoying!