Preheat oven to 425°F. Line a large baking sheet with parchment paper.
Arrange the crackers, salted side down, in a single layer on the lined baking sheet. Set aside.
To a small saucepan over medium-high heat, add the butter and brown sugar. Bring the mixture to a boil, stirring constantly. Allow the mixture to boil for 3-5 minutes, or until the butter is completely melted and the sugar has dissolved.
Add 1 teaspoon of hot cocoa mix and stir to combine. The toffee should reach between 270°F and 290°F (hard crack stage) before being poured over the crackers. It should darken in color and thicken slightly.
Remove the toffee mixture from the heat. Carefully pour it evenly over the saltine crackers, spreading it to the edges if needed.
Bake for 3-5 minutes, or until the butter mixture begins to bubble.
Carefully remove the pan from the oven and immediately sprinkle the chocolate chips on top of the crackers. Allow the chocolate chips to sit for about one minute or until they start to soften and melt.
Using an offset spatula, spread the melted chocolate evenly over the crackers.
Immediately sprinkle the hot cocoa powder and dehydrated mini marshmallows over the melted chocolate. Gently press the marshmallows into the chocolate to help them stick.
Transfer the pan to the refrigerator for about an hour, or until the mixture is set. Alternatively, you can place it in the freezer for quicker setting, for about 30 minutes.
Once set, break the Hot Cocoa Christmas Crack into small, uneven pieces by hand.