Preheat oven to 350°F. Line a large baking sheet with parchment paper or silicone baking mat.
In a large mixing bowl, use electric mixer to cream together softened butter and both sugars on medium speed until light and fluffy, about 2 minutes.
Add eggs and vanilla extract, beating on low until incorporated.
In a medium bowl, whisk together flour, hot chocolate mix, cocoa powder, baking soda, and salt.
Add dry ingredients to wet ingredients and beat until just combined. Fold in 1 cup of chopped chocolate and 1/2 cup mini marshmallows.
Use a medium cookie scoop to portion out 2 tablespoons of dough. Roll into balls with your hands and place on lined baking sheet, spaced at least 2 inches apart.
Bake for 8-10 minutes, until edges just start to brown. Do not overbake.
Remove from oven and cool on baking sheet for 5 minutes. Press remaining chocolate and marshmallows into tops of warm cookies.
Transfer to wire cooling rack to cool completely.
Notes
Bring butter to room temperature for proper creaming. Don’t overbake – cookies continue cooking while cooling.
Use unsweetened natural cocoa powder to avoid over-sweetening. If using salted butter, omit the salt in recipe.