Preheat the oven to 350°F. Line two baking sheets with parchment paper. Set aside.
In a medium sized bowl use a whisk to sift together flour, hot chocolate mix, cocoa powder, baking soda, salt and espresso powder, if using. Set aside.
In a large mixing bowl, use an electric mixer on medium high speed to beat together 2/3 cup granulated sugar with butter and vanilla extract until creamed, about 3-4 minutes. Add eggs one at a time mixing well after each addition. Stop and scrape the sides of the bowl periodically.
Lower the speed of the mixer and gradually add the sifted dry ingredients beating just until fully combined.
Place reserved 1/3 cup sugar in a shallow bowl.
Use a small scoop (meatball size) to divide the dough rolling into 1 inch balls. Roll the balls in sugar. Place on the prepared pans at least 2 inches apart.
Bake until set, about 8-10 minutes. Remove from the oven and immediately press 3 mini marshmallows into the center of each cookie. Watch your fingers, the cookies will be hot.
Change the oven setting to broil. Put the cookies under low broil heat just until the marshmallows are lightly browned, approximately 1-2 minutes. You can also use a pastry torch.
Remove from oven and immediately sprinkle with chocolate sprinkles. Cool on the pan for 5 minutes then transfer to wire racks to cool.
Enjoy warm or at room temperature, storing in an airtight container for up to 3 days.