Preheat the oven to 350°F and line two baking sheets with parchment paper.

Beat the butter and sugar together for 2 minutes, until pale and fluffy.

Beat in the egg and vanilla until smooth.

Whisk the flour, baking soda, and salt together in a separate bowl, then stir this into the butter mixture just until no dry streaks remain.

Fold in 1 cup of the marshmallow bits, the dough will be soft and a little sticky, that is correct.

Scoop the dough into 12 balls, about 2 tablespoons each, and set them 2 inches apart on the prepared sheets.

Press a few of the remaining marshmallow bits onto the top of each ball so they show once baked.

Bake for 9 to 11 minutes, until the edges are set and lightly golden but the centers still look slightly underdone, they firm up as they cool.

Cool the cookies on the pan for 5 minutes before moving them to a wire rack, they are too soft to lift straight from the oven.