In a large mixing bowl, cream together ½ cup of unsalted butter, ½ cup granulated sugar, and ½ cup brown sugar using an electric mixer on medium speed until light and fluffy, about 3-5 minutes.

Add in 1 large egg and 1 teaspoon of vanilla extract to your creamed mixture. Beat until well combined.

Gradually sift in 1 cup of all-purpose flour and ½ cup of unsweetened cocoa powder along with a pinch of salt. Mix on low speed until just combined.

Gently fold in about 1 cup of chopped marshmallows into the thick cookie dough.

If your dough feels sticky, wrap it in plastic wrap and refrigerate for about 30 minutes.

Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper and scoop tablespoon-sized portions of the dough onto the sheets.

Bake for 10-12 minutes, rotating the sheets halfway through.

Once baked, remove the cookie sheets from the oven and let them cool for about 5 minutes on the sheets.