1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a large mixing bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.
3. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.
4. In another bowl, whisk together the flour, cocoa powder, baking soda, and salt.
5. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
6. Fold in the chocolate chips and mini marshmallows. Be gentle to avoid squishing the marshmallows too much.
7. Scoop tablespoon-sized amounts of dough onto the prepared baking sheet, leaving space between each cookie to allow for spreading.
8. Bake in preheated oven for 10-12 minutes, or until the edges are set but the centers are still soft. Remember, the cookies will continue to cook on the baking sheet after you remove them from the oven.
9. Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.

A little tip from my mum: If you’re baking on a particularly cold day, let the dough sit at room temperature for about 10 minutes before baking to ensure even cooking.