Line an 8-inch or 9-inch square pan with foil. Spray lightly with cooking spray and lightly wipe with paper towel. Set aside.
Combine chocolate chips, half of sweetened condensed milk and 1 tablespoon of butter in a microwave-safe bowl.
In a another microwave-safe bowl, combine peanut butter chips, remaining half of sweetened condensed milk and 1 tablespoon of butter.
Heat both bowls on high for 30 seconds, stir, repeat until chips are melted and smooth. Do one at a time if both bowls don't fit in your microwave.
Add one teaspoon of vanilla into each bowl and stir to combine. The mixtures should be smooth and glossy.
Spoon both the chocolate and peanut butter mixtures into the prepared pan alternating flavors so that all the chocolate isn't together. See video for reference.
Sprinkle on half of the marshmallows and all of the nuts.
Use a spatula or knife to cut through the fudge, evenly distributing the marshmallows and nuts and creating a swirl pattern with the chocolate and peanut butter fudge.
Top with remaining marshmallows and press down gently to secure.
Let cool to room temperature before transferring to refrigerator to chill.
Remove from pan, peel off foil, and cut into squares.
Store in an airtight container.