Prep your pan: Line an 8x8-inch square pan with parchment paper, leaving overhang on two sides to lift the fudge out later. Lightly butter or spray the parchment.

Set up a gentle melt: In a medium, heavy-bottomed saucepan, add the sweetened condensed milk and butter. Warm over low heat, stirring often, until the butter melts and the mixture looks glossy.

Add the chocolate: Turn the heat to the lowest setting. Stir in the milk chocolate chips. Keep stirring slowly and steadily until fully melted and smooth. Do not let it simmer. If needed, take the pan off the heat and let residual warmth finish the melt.

Flavor and finish: Stir in vanilla and kosher salt until evenly combined. If using mix-ins like nuts or mini marshmallows, fold them in now.

Optional swirl: Pour the fudge into the lined pan and smooth the top. Warm the peanut butter for 10–15 seconds in the microwave, then dollop small spoonfuls over the surface. Use a butter knife to gently swirl. Sprinkle flaky salt if you want a sweet-salty finish.

Set the fudge: Let the pan sit at room temperature for 15–20 minutes, then transfer to the fridge for 2–3 hours, or until firm enough to slice.

Slice cleanly: Lift the fudge out using the parchment overhang. Warm a sharp knife under hot water, dry it, then make straight-down cuts into 1-inch squares. Wipe and rewarm the knife as needed for neat edges.