Line an 8x8 inch baking pan with parchment paper. Coat the parchment paper with cooking spray and set aside.
In a medium saucepan, bring the light brown sugar, unsalted butter, and evaporated milk to a boil while stirring frequently. Once boiling, boil for 5 minutes, stirring constantly.
Remove from heat and stir in the peanut butter until it is fully incorporated.
Take the peanut butter mixture and thoroughly mix it together with the marshmallow fluff, vanilla extract, lemon juice, and salt.
Once mixed, evenly spread the mixture into your prepared pan and refrigerate for 4 hours to overnight or until the fudge has hardened.
Remove fudge from the pan using the parchment paper, cut, and serve.