Preheat your oven to 350°F (175°C) to ensure even baking.
In a microwave-safe bowl, combine the chopped semi-sweet baking chocolate and unsalted butter. Microwave in 30-second intervals, stirring in between, until melted and smooth. Set aside to cool slightly.
In another bowl, whisk together the flour, cocoa powder, baking powder, and salt.
In a large mixing bowl, beat together the brown sugar, granulated sugar, and eggs until light and fluffy. Add the vanilla extract and mix until well combined.
Gradually pour the melted chocolate mixture into the sugar and egg mixture, stirring until well incorporated. Then, gradually add the dry ingredients, mixing until just combined.
Gently fold in the walnut pieces, half of the mini marshmallows, and half of the chopped milk chocolate bars.
Using a cookie scoop or spoon, drop tablespoon-sized portions of dough onto a lined baking sheet, leaving space between each cookie.
Press a few of the remaining mini marshmallows and chopped milk chocolate bars onto the top of each cookie dough mound.
Bake for 10-12 minutes, or until the edges are set but the centers are slightly soft. They will firm up as they cool.
Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
