Line an 8-inch square pan with parchment, leaving overhang. Lightly butter the paper.
Combine condensed milk, butter, and chocolate chips in a medium saucepan.
Warm over low heat, stirring constantly, until the chocolate melts and the mixture is smooth.
Remove from heat and stir in peanut butter until glossy and fully blended.
Mix in vanilla and salt. Fold in chopped peanuts if using.
Scrape into the pan and smooth the top. Tap the pan to release air bubbles.
Chill until firm, about 2–3 hours. Lift out, slice into 16 squares, and serve.