1In a medium saucepan, melt the butter over medium heat.
Add the peanut butter, sweetened condensed milk, and brown sugar to the melted butter. Stir continuously until the mixture is smooth and well combined.
Remove the saucepan from the heat and stir in the vanilla and salt.
Pour the mixture into a greased or parchment-lined 8x8 inch baking dish.
Allow the fudge to cool at room temperature for about 1 hour, then refrigerate for at least 2 hours or until firm.
Once the fudge is set, cut it into squares and serve.