First, start by preheating the oven to 350F, then line a large baking sheet with parchment paper.
In a stand mixer fitted with a paddle attachment, cream the butter, granulated sugar, brown sugar, and both peanut butters. Continue mixing until it turns light and fluffy.
Add in the eggs, vanilla, and the room temp water and continue mixing until everything is well incorporated.
Slowly add in the flour, baking soda, cream of tartar, and salt and continue mixing on low speed, scraping the sides as you go. Fold in the chocolate chips and the mini marshmallows.
Using a medium ice cream scoop, scoop round balls onto the cookie sheet and bake for 8-9 minutes.
Remove the tray from the oven and let the cookies cool for 10 minutes and then transfer to a wire rack.
These cookies can be stored in an airtight container at room temperature for 3 days, or in the fridge for up to 6 days. Just make sure to pull them out an hour before eating them to let them soften and get back to room temperature.