Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper and set aside.
Cream the butter, brown sugar, and granulated sugar together until light and creamy.
Add in the egg, egg yolk, heavy cream, and vanilla extract, mixing until well combined.
In a separate bowl, sift together the flour, cocoa powder, and baking soda. Gradually add the dry mixture to the wet ingredients, mixing just until the dry ingredients disappear.
Fold in the milk chocolate chips evenly.
Use a cookie scoop to portion out 8 cookie dough balls, then gently flatten each ball to about ½ inch thick.
Bake for 10 minutes in the preheated oven. Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack.
While the cookies are cooling, make the frozen hot chocolate mousse. In a stand mixer, whip the heavy cream, hot cocoa mix, and cocoa powder until fluffy and thick (about 3-4 minutes).
Once the cookies are completely cooled, frost each cookie with the frozen hot chocolate mousse, then top with mini marshmallows for decoration.