Preheat your oven to 350°F. 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, beat the softened butter, granulated sugar, and brown sugar together until creamy. Add the vanilla extract and eggs, one at a time, mixing well after each addition.

Gradually add the dry ingredients to the wet mixture, mixing until just combined. Fold in the chocolate chips and mini marshmallows.

Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Drizzle the chocolate syrup over the tops of the cookie dough balls. Using a toothpick or the back of a spoon, swirl the syrup into the dough for a marbled effect.

Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. Remove from the oven and let cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely.