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

In a medium bowl, whisk together the all-purpose flour, baking soda, and salt.

In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and brown sugar using an electric mixer until light and fluffy.

Add the egg and vanilla extract, blending until well combined.

Gradually mix the dry ingredients into the wet mixture, stirring until just combined.

Gently fold in the Rice Krispies, chocolate chips, and mini marshmallows.

Drop rounded tablespoons of dough onto the prepared baking sheet, leaving space between them.

Bake for 10-12 minutes, until the edges are golden brown and centers look slightly underbaked.

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