Preheat & Prep: Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
Mix Dry Ingredients: In a medium bowl, whisk together flour, baking soda, and salt.
Cream Butter & Sugar: Beat butter and brown sugar until fluffy.
Add Wet Ingredients: Add egg and vanilla extract to the mixture.
Add Dry Ingredients: Gradually mix in the dry ingredients until fully combined.
Stir in Chocolate Chips & Marshmallows: Gently fold in chocolate chips and mini marshmallows.
Scoop Dough: Drop spoonfuls of dough onto the baking sheet, spacing them out.
Bake: Bake for 10-12 minutes, until golden. Let cool on a wire rack.
Make Espresso Swirl: Dissolve espresso powder in hot water, and stir in powdered sugar to create a smooth glaze.
Drizzle: Once cookies have cooled, drizzle the espresso swirl over each cookie.