In a heavy saucepan, over low heat, melt peanut butter chips with 1 cup of sweetened condensed milk, butter, and salt.

In a small saucepan, over low heat, melt chocolate chips with the remaining sweetened condensed milk. Remove from heat and add vanilla extract. Stir until smooth.

Spread the peanut butter mixture evenly into a waxed paper-lined 8- or 9-inch square pan.

Spoon the chocolate mixture over the peanut butter mixture. Use a table knife or metal spatula to swirl the chocolate through the top of the fudge.

Chill in the refrigerator for 3 hours or until firm.

Turn fudge onto a cutting board, peel off the paper, and cut into squares.