Preheat the Oven: Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper or silicone baking mats.
Mix Wet Ingredients: In a large bowl, cream together the softened butter, sugar, and brown sugar until light and fluffy. Beat in the eggs, one at a time, followed by the vanilla extract.
Combine Dry Ingredients: In a separate bowl, whisk together the flour, hot cocoa mix, salt, baking soda, baking powder and optional cocoa powder.
Mix Dough: Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Stir in the chocolate chips.
Chill the Dough: Cover the dough and chill it in the refrigerator for at least 1 hour (or up to 24 hours) to make it easier to handle and enhance the flavor.
Portion the Dough: Scoop the chilled dough onto the prepared baking sheets. Use about 4 tablespoons of dough (1/4 cup) for large cookies or 2 tablespoons for smaller cookies. Leave space between the cookies as they will spread slightly.
Bake: Bake the cookies for 9–11 minutes for smaller cookies and 11–14 minutes for larger cookies, or until the edges are set and the centers look soft but not raw.
Add Marshmallows: Remove the cookies from the oven and immediately press several mini marshmallows onto the top of each cookie. Return the cookies to the oven for 1–2 minutes, just until the marshmallows are puffed and lightly toasted.
Cool and Serve: Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.