Preheat oven to 375 degrees F. Line two baking sheets with parchment paper.
In a large bowl of a stand mixer, cream butter and brown sugar together until light, about 3 minutes.
2/3 cup salted butter, 1 1/2 cups brown sugar
Add eggs and vanilla and mix well.
2 large eggs, 2 teaspoons vanilla
Add flour, cocoa powder, baking soda, and salt, mixing on medium-low speed until combined.
1 1/2 cup all-purpose flour, 1/3 cup cocoa powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
Stir in chocolate chips, almonds, and marshmallows. Scoop balls of cookie dough onto the parchment-lined baking sheet using a medium cookie scoop.
1 cup chocolate chips, 1 cup almonds, 1 cup mini marshmallows
Bake for 9-11 minutes, until mostly set, but still underbaked in the middle. Remove from the oven and immediately place a few extra marshmallows on top of each cookie, along with a few extra chopped almonds and chocolate chips, if desired. Return to the oven and bake for 1 minute longer, until marshmallows have puffed slightly.
Cool for 2 minutes on the baking sheet, then transfer to a wire rack to finish cooling. Store in an airtight container on the counter for up to 5 days.