In the bowl of a stand mixer fitted with a paddle attachment or in a large bowl with an electric hand mixer, cream unsalted butter, light brown sugar, and granulated sugar until light and fluffy.
Add eggs and vanilla extract and mix until smooth.
Add all-purpose flour, cocoa powder, baking soda, and salt. Mix on low until cookie dough forms.
Fold in chocolate chips and dehydrated marshmallows.
Portion the cookie dough with a medium cookie scoop. Place the cookie dough balls on a baking pan and chill for about 15 minutes.
Preheat oven to 350° F and prepare a baking sheet pan with parchment paper or a silicone baking mat.
Place cookie dough balls on the prepared baking sheets and allow space for spreading. Bake for approximately 8 to 10 minutes until the cookies are set on the edges but still appear soft in the center.
Remove the pan from oven. Cool the cookies on the pan for a minute or two. Transfer to a wire rack to cool completely.