Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.

In a large mixing bowl, cream together unsalted butter, granulated sugar, and brown sugar until light and fluffy.

Add large eggs one at a time, mixing thoroughly. Then stir in vanilla extract until smooth.

In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually add to the creamed mixture and mix just until combined.

Fold in chocolate chips and chopped walnuts until evenly distributed throughout the dough.

Using a cookie scoop, drop heaping tablespoons of dough onto prepared baking sheets, spacing them 2 inches apart.

Bake for 8 minutes, then press mini marshmallows into the centers of cookies. Return to oven and bake for another 2-3 minutes.

Remove from oven and let cool slightly. Drizzle melted chocolate on top of each cookie.

Allow cookies to cool completely on a wire rack before serving.