In a large mixing bowl, add in the butter and sugar. Beat on medium with hand mixer until creamed.
Add in the eggs and vanilla extract. Beat until incorporated. Set aside.
In a small mixing bowl, add in the flour, hot cocoa mix, baking powder, and cornstarch. Whisk to combine.
Add the dry ingredients into the wet and mix until the dough becomes to come together. It will look too dry at first, but keep mixing until it starts clumping together. At this point, you can use your hands and push the cookie dough together.
Add the cookie dough between two pieces of parchment paper and roll out to about ¼" inch thick.
Place into the fridge to chill for at least one hour. The longer, the better. I kept mine in there overnight.
Preheat oven to 350 degrees. Line baking sheet with parchment paper.
Cut the cookies out using the ugly sweater cookie cutter.
Place onto the prepared baking sheet.
Bake for 16 - 17 minutes until the edges start to brown up slightly. For smaller cutters, it will be less time so keep an eye out when using different cookie cutters than the one I used.
Let cool for about 10 minutes on the baking sheet and transfer to a cooling rack to cool completely.
Flooding/Stiff Icing
In a medium mixing bowl, add in the powdered sugar, corn syrup, milk, and vanilla. Using a spatula, stir until thoroughly incorporated.
For the flooding icing, spoon a little out and drizzle it on top of the bowl of icing. It should melt into the icing after 20 seconds. If it doesn't add an additional teaspoon of milk.
If you want to use colors, separate the icing out into a couple bowl and color each one with gel paste food color.
Add the flooding icing into a piping bag and the stiff icing into a different piping bag fitted with a small round tip.
Outline the cookies with the stiff icing.
Cut a small hole into the tip of the flooding icing bag and fill the cookie. Use a toothpick to move the icing to cover holes.
To blend the colored icing in and move it around with a toothpick, pipe in on the cookies while the base layer of icing is still wet. Move it around with a toothpick to form designs.
Video
Notes
You can use unsalted butter and add ½ teaspoon of salt into the dry ingredients.