Line an 8x8-inch pan with parchment paper, leaving an overhang.
In a large microwave-safe bowl, combine condensed milk, chocolate
chips, 1/2 cup peanut butter, and butter.
Microwave in 30-second intervals, stirring with a silicone spatula each
time, until melted and smooth. Should take about 2 minutes total.
Stir in vanilla and a pinch of salt.
Pour into the prepared pan and smooth the top with an offset spatula.
Melt the remaining 1/4 cup peanut butter in a microwave safe bowl for
15–20 seconds until pourable. Pour into a piping bag or zip-top bag cut
the corner of the bag, and pipe 4 horizontal lines across the fudge
mixture. Use a butter knife to gently swirl the peanut butter, dragging up
and down vertically.
Refrigerate for at least 2 hours or until firm. Cut into squares and serve!