In a small microwave safe bowl, melt together 2 tablespoons of butter and the semi sweet chocolate chips. Microwave for one minute and stir, then heat again in 30 second increments until smooth. Allow to cool for 5 minutes.
In a large bowl combine the chocolate chip cookie mix and powdered hot chocolate, set aside. Using an electric hand mixer, blend 5 tablespoons of softened butter until smooth, then slowly incorporate the egg and melted chocolate mixture.
With the mixer on medium low speed, slowly add in the dry ingredients and mix until a dough forms. Wrap the dough in plastic and refrigerate 20 minutes, until firm.
Preheat oven to 350 degrees and line a baking sheet with parchment paper. Roll the chilled dough into 1 1/2 - 2 inch cookie balls and place on cookie sheet at least one inch apart, gently flattening the top of each dough ball. Cut the chocolate bar into roughly 1/2 inch squares. Finely chop one of the chocolate bar pieces and set aside.
Bake cookies for 10 minutes then remove from oven and top each cookie with a piece of chocolate and 4-5 mini marshmallows, pressing down gently into the cookie. Bake another 3-4 minutes. Remove cookies from oven and sprinkle with finely chopped chocolate.