In a large mixing bowl, cream together ½ cup softened unsalted butter, ¼ cup granulated sugar, and ¾ cup dark brown sugar with an electric mixer on medium speed until light and fluffy, about 3-4 minutes. Add 1 large egg and 1 teaspoon vanilla extract, mixing until well combined.

In a separate bowl, whisk together 1 ¾ cups all-purpose flour, ½ cup natural cocoa powder, ½ cup hot cocoa mix, 1 teaspoon baking soda, and ¼ teaspoon salt until well blended. Gradually add the dry mixture into the wet ingredients while mixing on low speed until a smooth dough forms.

If the dough feels too thick, add a splash of milk for a creamy consistency. Cover with plastic wrap and chill for at least 2 hours.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. Roll chilled dough into 1-inch balls and place on sheets, leaving space for spreading.

Bake for about 10 minutes until edges are set but centers look soft. Remove from the oven.

Press half a marshmallow into the center of each warm cookie. Return to oven for 2 minutes until marshmallows puff.

Remove cookies and flatten marshmallows with a spatula. Allow to cool, then melt 4 ounces of semi-sweet chocolate and drizzle over cooled cookies.