Preheat oven to 350F, line a baking sheet with parchment paper.
Stir flour, salt, and baking soda together and set aside.
Cream butter, peanut butter, and sugars together until light and fluffy, then add egg, and vanilla, and beat until smooth.
Stir in the flour mixture until well combined, fold in the chocolate chips.
Divide into seven balls onto the parchment-lined baking sheet.
Flatten each ball as shown in pictures, spread a bit of peanut butter in the center (optional, but extra tasty), and put a marshmallow in the center of each, then pinch cookie dough around the top. It’s okay if some marshmallow is showing. It’s more important for the cookie dough to be under the marshmallow than above (so it doesn’t melt and run out over the pan)
Bake in the center of the oven for 12 minutes, until the edges of the cookies are set. Let cool on the pan for at least ten minutes before transferring them to a cooling rack.