Preheat the oven to 350°F (180°C) and prepare a 9x13 pan by spraying with a non-stick spray or lining with parchment (let some hang over the side for easy removal!). Set aside.
Using a stand mixer, cream together the butter and sugars until light in color and fluffy in texture. This will take about 2 minutes.
1 cup (222g) salted butter, softened, 1 cup (246g) brown sugar, 1/3 cup (76g) granulated sugar
Scrape down the sides and add in the eggs, corn syrup and vanilla. Mix until thoroughly combined.
2 eggs, 1 Tbsp corn syrup, 1/2 Tbsp vanilla extract
Next, add the flour, cocoa powder, hot cocoa mix, corn starch, baking soda and salt. Mix until your chocolate cookie dough forms. The dough will be soft.
2 1/2 cups (380g) all-purpose flour, 2/3 cup (80g) dutch processed cocoa, 1/3 cup (46g) hot cocoa mix, powder only, 3 tsp corn starch, 1 1/2 tsp baking soda, 3/4 tsp salt
Add in the chocolate chips and fold in until incorporated.
3/4 cup (150g) milk chocolate chips
Press the dough into the 9x13 pan and bake at 350°F (180°C) for 15-17 minutes. The cookie bars should puff up and the edges will begin to crackle.
Pull the bars out of the oven and set down forcefully to allow the expanded air to deflate. This will create a very fudgy bar texture. Allow the cookie bars to cool completely.
Once cool, make the hot cocoa mousse. Add the heavy whipping cream to the bowl of a stand mixer (or a large bowl if using a hand mixer). Beat on medium-high speed until it begins to thicken.
1 1/2 cups (360 ml) heavy whipping cream
Reduce the speed and gradually add the powdered sugar, hot cocoa mix, and cocoa powder.
3/4 cup (80g) powdered sugar, 3/4 cup (105g) hot cocoa mix powder only, 1 1/2 Tbsp dutch processed cocoa
Once incorporated, increase the speed to high and whip until stiff peaks form. The mousse is ready when it holds its shape.
Spread the mousse onto the top of the cookie bars and then top with mini marshmallows.
mini marshmallows
Chill until ready to cut and serve. Enjoy!