Preheat your oven to 325°F / 170°C and line a baking sheet with parchment paper.
Break the dark chocolate into small pieces and melt in the microwave in 30-second increments until totally melted, stirring in between each round.
In a separate bowl, beat together the butter, white sugar and brown sugar until fluffy.
Add the melted chocolate and mix until combined.
Add the egg and vanilla extract and beat until combined.
Add in the flour, cocoa powder, espresso powder, baking soda and salt. Mix until a soft dough forms and there are no lumps of flour.
Pour in all the chocolate chips and mix until they are evenly distributed throughout the cookie dough.
Scoop large balls of cookie dough using an ice cream scoop or use a tablespoon and roll with your hands. Space them out evenly on the cookie sheet and press down on them gently to flatten them out just a little.
Put marshmallows on top of the cookies and push them into the cookie dough a little so they won't slide off. If you are using mini marshmallows, put them straight on. If you have full-sized marshmallows, cut them into quarters before pressing them into the top of the dough.
Bake for 17 minutes. The cookies will still be soft but they should be dry to the touch. If you want perfectly round cookies, use a large circular cookie cutter to 'scoot' the cookies into shape as soon as they come out of the oven. Leave to cool on the sheet for a few minutes before transferring to a cooling rack.
Notes
If the cookies don't spread into a perfect circle while baking, you can 'scoot' them into shape as soon as they come out of the oven by taking a large circular cookie cutter and gently running it in a circle around each cookie to press the edges into shape. I always recommend measuring your ingredients with a kitchen scale, rather than with measuring cups - it's much more accurate this way. I provide all my recipes in both cups and grams so you can choose what works best for you.