Preheat your oven to 350°F (175°C), and line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, salt, and espresso powder. Set aside.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until it’s light and fluffy.
Beat in the egg and vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
Fold in the chocolate chips, marshmallows, and nuts (if using) until evenly distributed in the dough.
Drop spoonfuls of the cookie dough onto the prepared baking sheet, spacing them about two inches apart.
Bake in the preheated oven for 10-12 minutes, or until the edges look set and the middles are soft.
Remove from the oven and let them cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Notes
Serve the cookies on a decorative plate, and drizzle with melted chocolate for an extra touch. You can also dust them lightly with powdered sugar for a whimsical look!