Preheat the oven to 350°F. Grease a 9×5-inch loaf pan and line it with parchment paper, leaving enough parchment over the sides to lift the bread from the pan after baking. I never use parchment paper for my quick breads and have had no issues. Therefore, this step is optional.
In a medium bowl, whisk together the flour, cocoa powder, hot cocoa mix, baking soda, baking powder, and salt until evenly combined.
In a large mixing bowl, whisk the melted butter, granulated sugar, and brown sugar together. Add the eggs and vanilla and whisk until smooth.
Mix in the sour cream and milk until combined.
Add the dry ingredients to the wet ingredients and gently fold together just until you no longer see streaks of flour. Avoid overmixing.
Fold in ¾ cup chocolate chips.
Gently fold in ¾ cup mini marshmallows last. Fold only a few times so the marshmallows are distributed throughout the batter without being crushed.
Transfer the batter to the prepared loaf pan and smooth the top.