Begin by preheating your oven to 350°F (175°C). Grease a 12-cup metal bundt pan generously with butter or non-stick spray.
In a large mixing bowl, beat 1 ½ cups of room-temperature butter, 1 cup of granulated sugar, and ¾ cup of brown sugar using an electric mixer on medium speed until light and fluffy, about 3-5 minutes. Incorporate the 5 large eggs one at a time, then add 1 tablespoon of vanilla extract and ½ cup of cooled hot chocolate, blending until just combined.
In a separate bowl, whisk together 1 teaspoon of kosher salt, 2 teaspoons of baking powder, 1 cup of Dutch-processed cocoa powder, and 2 cups of all-purpose flour. Gradually mix the dry ingredients into the wet batter using low speed until just combined.
Pour the batter into the prepared bundt pan and bake for 50-60 minutes. Check doneness with a toothpick. Allow the cake to cool in the pan for about 10 minutes before inverting it onto a wire rack.
Prepare the glaze by combining 2 cups of confectioners' sugar, 1 packet of hot chocolate mix, and 2-3 tablespoons of water in a mixing bowl. Stir until smooth. Drizzle the glaze over the cooled cake.
Heat ½ cup of marshmallow fluff in the microwave for about 15 seconds and drizzle over the glazed cake. Sprinkle with ½ cup of miniature marshmallows.