Preheat the oven to 350°F. Use parchment or Silpat on the baking sheet.

Combine flour, oats, baking soda, and salt in a small container.

In a large bowl, mix peanut butter, the sugars, and melted butter on high speed with a hand mixer until fluffy, about 2 minutes. Add eggs and vanilla, then mix until completely combined. Add the flour mixture and mix on low speed just until combined.

Scoop cookie dough with a #40 scoop, roll into a ball, and bake 12 cookies per cookie sheet in a 350°F oven for 8 minutes. Remove the pan from the oven and place a half marshmallow in the middle of each cookie, pressing it in slightly. Return the cookies to the oven for 2 minutes.

Allow the cookies to cool completely on the cookie sheet. Melt the peanut butter chips and coconut oil in the microwave for approximately 30 seconds, until smooth. Using a small spoon, drizzle melted peanut butter chips over the marshmallows until fully covered, then let them set for several minutes before eating.