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

In a large bowl, cream together the softened butter and sugar until light and fluffy.

Add the egg and vanilla extract, mixing until fully combined.

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

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

Gently fold in the mini marshmallows.

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

Bake for 8-10 minutes, or until the edges are golden but the centers are still soft.

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

Enjoy your warm, gooey butter melted cookies with marshmallows!