Step 1: Prepare the Dough

In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar until light and fluffy.

Mix in the eggs one at a time, followed by the vanilla extract, until well combined.

Step 2: Combine Dry Ingredients

In another bowl, whisk together the flour, baking soda, and salt.

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

Step 3: Fold in Mix-Ins

Gently fold in the chocolate chips, mini marshmallows, and walnuts until evenly distributed.

Step 4: Shape the Cookies

Preheat your oven to 350°F (175°C).

Using a large cookie scoop or your hands, portion out the dough and shape it into thick balls (about 1/4 cup each) and place them on a baking sheet lined with parchment paper.

Step 5: Bake

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

Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.