Line an 8 x 8 baking dish with parchment paper and lightly spray with non-stick spray. Set aside.
To a medium sized sauce pan, add the milk and marshmallows and heat over medium high heat until the marshmallows have completely melted.
Reduce heat to medium and add the sugars, vanilla extract, and salt. Stir until completely incorporated and sugar no longer appears grainy.
Add the peanut butter and cook for two more minutes.
Remove from heat and pour the fudge out into your prepared pan.
Refrigerate for 4 hours or overnight before cutting into 1" squares.
You can also make this in a 9x9 pan, but it will be thinner, or an 8x4 loaf pan and it will be thicker.Store in an air tight container for up to 3 months.If you don't want to use the stove, this can also be made in the microwave in a microwave safe bowl.