Preheat the oven to 350 F. Line 2 baking sheets with parchment paper or silicone baking mats.
Microwave the butter in a large, microwave-safe bowl for about 40 seconds. Butter should mostly melted.
Whisk the butter until it is completely melted.
Add the sugars; mix until well-combined.
Stir in vanilla and egg until incorporated.
Add the flour, cocoa powder, baking soda, and salt. Please read the recipe note about properly measuring flour.
Mix dough until just combined. Dough should be soft and a little sticky but not overly sticky.
Stir in chocolate chips, mini marshmallows, and sliced almonds.
Scoop out 1.5 tablespoons of dough (medium cookie scoop) and place 2 inches apart on baking sheet.
Bake for 7-10 minutes, or until cookies are set. They will be puffy and still look a little underbaked in the middle.
Notes
Butter: This recipe uses melted butter. I prefer to use salted butter, but you can use unsalted butter if you prefer.
Measuring Flour: If you aren’t weighing your ingredients, I can’t recommend that enough. If you are using measuring cups, be sure to sift or stir the flour before measuring. Flour packs easily and using too much flour in cookies will give you dry, crumbly dough and cookies that don’t spread.