Melt 1 cup of unsalted butter in a medium saucepan over medium heat, stirring constantly until it turns golden brown, about 5-7 minutes.

In a large mixing bowl, combine the cooled brown butter with 1 cup of brown sugar and 0.5 cup of granulated sugar. Blend until creamy and smooth.

Beat in 2 large eggs, one at a time, then stir in 1 teaspoon of vanilla extract.

In a separate bowl, whisk together 2.5 cups of all-purpose flour, 1 teaspoon of baking soda, and 0.5 teaspoon of salt, then gradually add to the wet ingredients until just combined.

Fold in 2 cups of mini marshmallows until evenly distributed.

Cover the bowl and refrigerate the dough for at least 30 minutes.

Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop mounds of dough onto the prepared sheets, leaving space between each.

Bake for 10-12 minutes, or until edges are golden brown.

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