Preheat the oven to 350°F. Line a baking sheet with parchment paper and set aside.
In a stand mixer fitted with the paddle attachment, cream together the butter and sugars for about 2 minutes until light and fluffy. Scrape down the sides of the bowl and add in the egg and vanilla extract, mixing until fully combined.
In a separate bowl, sift together the flour, cocoa powder, espresso powder, baking soda, and salt. Gradually add the dry ingredients to the butter mixture, stirring until just combined. Be careful not to over-mix.
Fold in the chocolate chips gently.
Add 1/4 of the marshmallow fluff into the dough and fold it in just once or twice, so it stays marbled. Use a large cookie scoop to scoop 1-inch dough balls, making sure to include some marshmallow fluff in each scoop. Place the cookie dough on the prepared baking sheet about 2 inches apart.
Repeat this process, adding marshmallow fluff in increments as you scoop the dough, ensuring each cookie has a nice swirl of fluff.
Bake for 10-12 minutes, keeping an eye on the cookies to ensure they bake with a soft center and firmer edges.
Let the cookies cool on a rack for 10 minutes before serving. Enjoy!