Step 1

Preheat the oven to 375 degrees F.

Step 2

Line two baking sheets with parchment paper or silicone baking mats.

Step 3

Melt the butter in a heavy saucepan.

Step 4

Add the brown sugar and the granulated sugar to a large bowl.

Step 5

Pour the melted butter over the sugar, stirring to combine, then let stand for 5 minutes.

Step 6

Add the egg and the egg yolk to the sugar mixture, stirring to combine.

Step 7

Stir the vanilla into the sugar mixture.

Step 8

Add the flour, the cocoa powder, the baking soda, and the salt to the sugar mixture, stirring until just combined.

Step 9

Stir the chopped chocolate and the marshmallow bits into the dough mixture.

Step 10

Chill the dough in the fridge for at least 30 minutes and up to 2 days. This allows the flavors to meld and the butter to chill, improving the flavors.

Step 11

Drop 1 1/2-tablespoon-sized scoops onto the prepared cookie sheet, keeping them about 2 inches apart.

Step 12

Bake until the cookies are set and slightly underdone in the center, about 8-12 minutes.

Step 13

Let the cookies cool on the cookie sheet for 5 minutes, then allow them to cool completely on a wire rack.

Step 14

Serve.