Using a stand mixer, cream butter, sugars and vanilla together.
Add in egg and combine
In a separate bowl, combine flour, corn starch, baking powder, salt and hot chocolate
Slowly add the flour mixture into the wet ingredients
Note: Dough will be very thick. You may need to finish combining the ingredients by hand if your mixer cannot handle the thickness of the dough
Add in chocolate chips and combine
Chill the dough for a full hour
Preheat oven to 350 F
Scoop a small amount of dough into your hands and roll into a ball. Create a "nest" in the center of each ball of dough and place on a greased cookie sheet
Add a few marshmallows into each nest
Flatten a small piece of dough with your hands big enough to cover the nest, and gently press each cookie down to flatten it a little
Bake for 12 minutes, or until the edges are golden brown
Note: The cookies will still look very soft when you take them out of the oven. That's normal - they will continue to cook as they are cooling
Serve with a glass of hot chocolate and enjoy!
Yields 1 dozen