Preheat the oven to 350 degrees Fahrenheit. Prepare cookie sheets by lining them with nonstick baking mats or spraying them lightly with nonstick cooking spray.

In the bowl of an electric mixer, cream together softened butter, brown sugar, granulated sugar, vanilla, eggs, and peanut butter. Cream it well, scraping down the sides in between, until the batter lightens up.

In a separate bowl, whisk together flour, baking soda and salt. Add flour mixture to wet mixture and mix until incorporated. Mix in white chocolate chips. Stir in frozen marshmallows by hand (keep the marshmallows in the freezer until you're ready to stir them into the dough).

Scoop or form dough into 1 1/2" to 2" balls and place on prepared pan at least 2" apart.

Bake for approximately 9 minutes. Be careful not to over bake.

Allow cookies to cool on the cookie sheets for at least 5 minutes before trying to remove them. The marshmallows will want to stick to the pan, so to avoid that, wipe your spatula clean with a damp cloth in between removing each cookie.