In a medium mixing bowl, whisk together the All-Purpose Flour (2 3/4 cup) (plus 2 Tbps), Unsweetened Cocoa Powder (1 tablespoon), Hot Chocolate Mix (2/3 cup), Corn Starch (2 tablespoon), Baking Soda (1 teaspoon) and Salt (1 teaspoon); set aside.
In the bowl of an electric stand mixer fitted with the paddle attachment, cream together the Unsalted Butter (1 cup), Granulated Sugar (1 cup) and Brown Sugar (1/2 cup) until well blended.
Add in the Egg (2) one at a time, then blend in Pure Vanilla Extract (2 teaspoon). Add half of the flour mixture at a time, and mix on low speed just until combined; add remaining flour mixture and mix until just combined.
Stir in Milk Chocolate Chips (1 1/4 cup). Cover and refrigerate overnight.
The next day, preheat oven to 350 degrees F (175 degrees C) and line 2 baking sheets with silicone liners or parchment paper.
Scoop dough out and shape into balls, about 1-1/2 tablespoons for each cookie. Space cookies 3 inches apart on prepared pans.
Bake one sheet at a time for about 11-12 minutes. Edges should be set (cookies will appear puffy and slightly under-baked).
Remove from oven and immediately place 4 to 5 Mini Marshmallows (1 1/2 cup) on top of each cookie; return to oven and bake until marshmallows are slightly melted (about 1-1/2 minutes).
Allow to cool on pans for 5 minutes. Cool cookies completely before proceeding to next step.
Place Milk Chocolate Chips (1 cup) and Vegetable Oil (1/2 tablespoon) in a small microwave-safe bowl and heat in the Panasonic Microwave Oven in 30-second increments, stirring every 30 seconds, until chocolate is melted; stir well to combine.
Carefully drizzle melted chocolate on top of each cookie. Store the cookies in an airtight container at room temperature for up to 3 days. Cooking results may vary depending on microwave oven used.