First Step: Preheat Oven and Prepare Baking Sheets
Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper for easy cleanup and to prevent sticking. Optionally, lightly grease or spray the parchment paper with cooking spray if desired.

Second Step: Mix Dry Ingredients
In a large mixing bowl, whisk together 2 ½ cups powdered sugar, ¾ cup unsweetened cocoa powder, and ½ teaspoon salt until well combined. This ensures even distribution of flavors and helps form your fudgy base.

Third Step: Add Egg Whites and Vanilla
Beat in 3 large egg whites and 1 to 2 teaspoons of vanilla extract to the dry mixture. Stir continuously until the batter forms a smooth and thick, brownie-like consistency. Add a fourth egg white only if necessary to achieve the right texture.

Fourth Step: Fold in Marshmallows and Chocolate Chips
Gently fold in 1 cup mini marshmallows and ½ to 1 ½ cups mini semi-sweet chocolate chips, depending on your chocolate preference. Evenly distributing these ingredients will ensure gooey and chocolatey pockets throughout the cookies.

Fifth Step: Rest the Dough
For optimal texture and shiny crackled tops, let the dough rest at room temperature for 15 to 20 minutes before scooping. This step improves the look and feel of your cookies.

Sixth Step: Scoop and Bake
Using a cookie scoop or spoon, drop heaping tablespoonfuls (about 1 ½ tablespoons) of dough onto the prepared baking sheets. Space cookies at least 2 inches apart to allow for spreading. Bake for 11 to 14 minutes or until cookies are puffed, glossy, and crackled on top. Avoid overbaking to keep centers soft and gooey.

Final Step: Cool and Serve
Allow cookies to cool on the baking sheets for at least 10 minutes to set, as they are fragile when warm. Transfer to a wire rack to cool completely. Serve immediately for that freshly baked gooey charm, or store properly for later enjoyment.