1. Preheat the oven to 375 degrees.
2. Place parchment or a silpat on a baking sheet.
3. In the bowl of a standing mixer, beat the butter and peanut butter for 1 minute.
4. Add the brown sugar and beat for an additional 2 minutes.
5. Add the milk, vanilla and egg and beat until smooth.
6. In a small bowl, whisk together the flour and baking soda, add to the wet ingredients and mix until smooth.
7. Add the chocolate chips, Reese's and marshmallows.
8. Scoop onto the cookie sheet and bake for 7 minutes.
9. Allow to cool slightly before removing to a cooling rack.
Store in airtight containers.