Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine the softened butter, white sugar, and brown sugar. Beat them together for about 3-4 minutes until light and fluffy.

Add the egg and two egg yolks one at a time, mixing well in between. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, hot chocolate mix, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in both the dark and semisweet chocolate chips.

Chill the dough in the refrigerator for 20 minutes.

Using a cookie scoop, place 1 1/2 to 2 tablespoons of dough onto a parchment-lined cookie sheet. Gently press them down.

Bake for 8-10 minutes until the edges are crisp and the centers remain soft.

Allow them to sit on the pan for a few minutes before transferring to a wire rack to cool completely.

Combine the softened butter, marshmallow cream, and powdered sugar in a bowl. Beat until smooth and fluffy.

Add the vanilla extract and whipping cream and mix until well combined.

Once the cookies are cool, pipe or spread the marshmallow buttercream on top and garnish with mini marshmallow bits.