Melt the semi-sweet chocolate bars in the microwave and let cool.
In a mixing bowl, combine four, cocoa powder, baking powder, and salt together. set aside.
In the bowl of a stand mixer, with the paddle attachment, beat butter and sugars together for 2 minutes. Add the eggs and vanilla and mix until smooth. Mix in the melted chocolate. Add the dry ingredients and mix until combined, scraping the sides as needed.
Use a wooden spoon or a spatula to mix in the walnuts, 1/2 cup marshmallows, and 1/2 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 oven to 350 degrees and line baking sheet with parchment paper or silpat mats.
Scoop dough and form into (24) 1 1/2 inch balls. Place 3 inches apart on pan. Bake for 11 minutes and then press 3 marshmallows into the tops of each. 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. Then move to a cooling rack.