Using a stand mixer, cream together the butter and sugars until light in color and fluffy in texture.
Add in the eggs, corn syrup and 1 tbsp of vanilla. Mix until thoroughly combined.
Next, add the flour, cocoa, hot cocoa mix, cornstarch, baking soda and salt. Mix until your chocolate cookie dough forms. The dough will be soft.
Fold in the chocolate chips, if desired.
Divide the dough out into 12 equal portions, I like to use my 1/2 cup measuring cup to help divide the dough. You'll want to use a little less than 1/2 cup of dough to have 12 cookies.
Roll each portion onto a ball then flatten slightly so it looks like a hockey puck before arranging on a parchment paper or silicone mat covered pan. Place 6 cookies per tray.
Bake at 350° for 15-17 minutes. The cookie should start to crack and crinkle, it will spread slightly and the centers of the cookie should no longer look wet before taking them out of the oven.
Allow the cookies to cool completely on the pan.
Once the cookies are cool, make the hot cocoa mousse.
Make the mousse by adding the heavy whipping cream to either the bowl of your stand mixer or a large bowl that you can use an electric hand mixer with.
Whip the cream on medium high speed until it starts to thicken.
Once it starts to thicken, slow the speed down and gradually add in the powdered sugar, hot cocoa mix and cocoa powder.
Once all of the powdered sugar has been added bump the speed up to as high as it can go and whip it until stiff peaks form. Once stiff peaks form, the frosting is done!
Divide the mousse among the cookies and then spread the topping until smooth.
Top with 5 mini marshmallows on each cookie.
Chill until ready to serve. Store in the fridge.
*I don't particularly love cold hard chocolate chips in my chilled cookie so I tend to leave them out. I have them in the recipe here because that is how Crumbl makes their cookies, with milk chocolate chips in them!