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

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

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

Add the egg and vanilla extract to the butter-sugar mixture and mix until fully combined.

Gradually add the dry ingredients to the wet mixture, stirring gently to combine without overmixing.

Fold in the Rice Krispies, chocolate chips, and optional mini marshmallows into the dough.

Using a cookie scoop, drop dough onto the baking sheet, spacing them about 2 inches apart.

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

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

Serve warm or store in an airtight container for up to a week.