Preheat the oven to 350°F. Line 3-4 baking sheets with parchment paper and set aside.
In a medium mixing bowl, mix together the flour, baking soda, and salt.
In a separate, large mixing bowl, beat the butter, peanut butter, canola oil, sugar, and brown sugar until creamy. Beat in the milk and vanilla.
Add the dry mixture into the wet and stir until just combined. Stir in the marshmallows.
Roll out 1 1/2 tbsp (25 g) balls of dough and place on the baking sheet, leaving room in between for spreading.
Bake for 9-11 minutes. Allow to cool slightly, then enjoy.