Start Creamy: In a big bowl, use a mixer to beat the softened butter with both granulated and brown sugars until the mixture looks light and fluffy, about 2-3 minutes. Then, mix in the egg and vanilla extract until everything is smooth and well combined.
Dry Mix Magic: Grab a separate bowl and whisk together your flour, natural cocoa powder, hot cocoa mix, baking soda, and salt. This step makes sure all the delicious dry ingredients are evenly distributed.
Combine & Chill (Don't Skip!): Slowly add the dry mixture into the wet ingredients, mixing on low speed until just combined. The dough will be thick and a little sticky – that’s perfect! Stir in the milk. Cover the dough tightly and let it chill in the refrigerator for at least 2 hours (or up to 3 days). Chilling is the secret to thick, bakery-style cookies that hold their shape beautifully!
Bake the Bases: Preheat your oven to 350°F (177°C). Line a large baking sheet with parchment paper. Scoop out heaping tablespoons of the chilled dough (about 25-26g each) and roll them into balls. Place them a few inches apart on your baking sheet. Bake for just 10 minutes; they'll look slightly underdone, which is exactly what we want for that gooey center!
Marshmallow Surprise Time: Immediately after taking the cookies out of the oven, gently press a halved marshmallow (cut side down) into the top of each warm cookie. Pop them back into the oven for just 2 more minutes until the marshmallows puff up. Once out, use the back of a spoon to gently flatten each marshmallow a bit.
Cool & Drizzle: Let the cookies cool on the baking sheet for 10 minutes, then carefully transfer them to a wire rack to cool completely. While they cool, melt your finely chopped semi-sweet chocolate in a microwave (stirring every 20 seconds until smooth) or using a double boiler. Spoon or drizzle this glossy melted chocolate over each cooled marshmallow-topped cookie. Let the chocolate set at room temperature until firm and shiny before stacking or serving. Enjoy this delightful treat responsibly!