Preheat oven to 350 degrees F. Line baking sheets with parchment paper or silicone liner.
In a large mixing bowl, cream together the butter, granulated sugar and brown sugar until light and fluffy. Beat in the eggs, one at a time, then blend in the vanilla.
In a separate medium bowl, whisk together the flour, hot cocoa mix, baking soda, salt and the sprinkles. Slowly add to the wet ingredients and mix just until combined, scraping the sides of the bowl as needed.
Drop six large dough balls onto the prepared cookie sheets and bake for 8-9 minutes. Remove the cookie sheet from the oven. Working quickly, carefully add one heart-shaped marshmallow to the center of each cookie, pressing lightly to adhere. Return to the oven and bake an additional 1 1/2 to 2 minutes, or until the marshmallows start to puff. Remove from oven, place on wire rack and gently press the warm marshmallow again to flatten the heart a little and stick to the cookie a little better. Allow to cool on cookie sheet for 10 minutes before removing to wire rack to cool completely.
Warning: It's hard to not eat all of the dough! :)