Prep the pan: Line an 8x8-inch baking pan with parchment paper, letting the paper hang over two sides for easy lifting. Lightly grease the parchment.

Make a gentle melt zone: In a medium saucepan, add the sweetened condensed milk, chocolate chips, and butter. Warm over low heat, stirring often, until the chocolate is almost melted and the mixture looks glossy.

Stir in the flavor: Remove from heat. Add the vanilla and salt. Stir until completely smooth.

Add the peanut butter: Stir in the peanut butter until the mixture turns silky and uniform. If you want a layered or swirled look, see the variations below before mixing fully.

Fold in extras (optional): If using nuts or other mix-ins, gently fold them in now.

Set it up: Pour the fudge into the prepared pan and smooth the top with a spatula. If you like, sprinkle a pinch of flaky salt.

Chill: Refrigerate for 2–3 hours, or until firm enough to cut cleanly. For neat squares, let it sit at room temperature for 5–10 minutes before slicing.

Slice and serve: Lift the fudge out using the parchment overhang. Cut into 1-inch squares with a warm, sharp knife, wiping the blade between cuts.