Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth.

Beat in the eggs one at a time, then stir in the vanilla extract.

In a separate bowl, combine the flour, baking soda, and salt. Gradually add this to the wet mixture until just combined.

Fold in the chocolate chips, mini marshmallows, and chopped walnuts into the cookie dough.

Drop rounded tablespoons of dough onto the prepared baking sheet, spaced about 2 inches apart.

Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.

Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack.