Butter or spray with cooking spray an 8x8 baking dish.

In large bowl, combine marshmallows, peanut butter chips and vanilla: set aside.

Mix milk, sugar and salt in large non-stick saucepan, over low heat.

When sugar is completely dissolved and not yet bubbling, add in 1/2 cup peanut butter and stir until melted.

Raise heat to medium and bring sugar/peanut butter mixture to boil stirring CONSTANTLY for 5 minutes to avoid scorching. (TIP: start timing when it starts to bubble around the edges.) This mixture turns a medium brown with the peanut butter and looks somewhat like caramel.

Remove from heat and immediately stir in marshmallows, peanut butter chips and vanilla from the bowl you set aside earlier.

Stir vigorously. It will get very thick. You will not be able to pour this fudge into your dish.

Scoop into prepared 8x8 baking dish.

Press down evenly with a rubber spatula. The fudge should be very thick and able to pull away from the sides. Make sure you press it down evenly.

Cool on wire rack for 10 minutes then transfer to refrigerator to finish cooling.

Cut into squares and enjoy!