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

In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the semi-sweet chocolate chips, milk chocolate chips, mini marshmallows, and walnuts (if using) until evenly distributed.

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

Bake for 12-15 minutes, or until the edges are golden brown but the centers are still soft.

Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.