Preheat the oven to 375 F. In a large mixing bowl with a hand mixer or in a stand mixer with a paddle attachment, cream together the butter and sugar until light and fluffy.
Add the egg and vanilla and beat until well combined.
Add in the flour, baking soda, and corn starch to the wet and mix until smooth.
Using a cookie scoop or tablespoon, form the dough into small balls and place into a greased mini muffin tin.
Chocolate Ganache Filling
Place chocolate in a heat-safe bowl and set aside.
In a saucepan over medium heat, bringing the heavy cream to a boil.
Stir in the hot cocoa mix.
Pour the hot cream over the semi-sweet chocolate chips and let it sit for 3 minutes. Then, stir until the chocolate chips are melted and the ganache is smooth.
Bake for 12-14 minutes at 375 degrees. Allow the cookies to cool completely in the pan before gently removing them
Break off the loops on some mini pretzels to use as handles.
Melt the white chocolate, transfer to a piping bag with a #3 tip and use it to attach the pretzel handles to the cookie cups.
Spoon the chocolate ganache into the cookie cups and top with mini marshmallows.
For best results, store the hot chocolate cookie cups in the refrigerator.