Line an 8-inch square pan with parchment paper or foil, leaving enough overhang on two sides to lift out the chilled fudge.

Place the peanut butter chips, white chocolate chips, and sweetened condensed milk in a saucepan over low heat. Stir constantly for 5 to 8 minutes, until completely melted and smooth, without allowing the mixture to boil.

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

Immediately spread the hot fudge mixture evenly into the prepared pan.

Refrigerate the pan for at least 2 hours, or until the fudge is firm.

Lift the fudge from the pan using the parchment or foil overhang. Cut into small squares with a large sharp knife, warming and wiping the knife dry between cuts for clean edges.