Line an 8x8 inch baking dish with parchment paper, leaving an overhang on the sides for easy removal.

In a medium saucepan over medium-low heat, combine the sweetened condensed milk, peanut butter, chocolate chips, and butter. Stir frequently until the butter is melted and the mixture is smooth.

Bring the mixture to a gentle simmer, stirring constantly for 5-7 minutes, until slightly thickened and glossy.

Remove from heat and stir in the vanilla extract and optional salt.

Pour the fudge mixture into the prepared baking dish and sprinkle with chopped peanuts (if using).

Refrigerate for at least 2 hours, or until firm.

Lift the fudge out of the dish using the parchment paper overhang. Cut into squares and serve.