Line an 8-inch-square baking pan with foil, pressing it into the corners and up the sides; leave an overhang on 2 sides. Brush the foil with butter. Combine 3/4 cup condensed milk, the chocolate chips and a pinch of salt in a medium saucepan. Combine the remaining condensed milk, the peanut butter chips and a pinch of salt in a separate saucepan. Put both pans over low heat and cook, stirring occasionally, until the chips are melted and smooth Spoon the chocolate mixture into the prepared pan, leaving spaces between the spoonfuls. Spoon the peanut butter mixture into the gaps. Brush an 8-inch square of parchment paper with butter and lay it directly on top of the fudge; press to flatten evenly and fill any gaps. Refrigerate until firm, about 45 minutes. Lift the foil to remove the fudge from the pan and cut into 1-inch squares. Store up to 1 week in an airtight container