Step 1: Prepare your pan. Line an 8x8 inch square baking pan with parchment paper, leaving an overhang on the sides. Lightly grease the parchment paper with butter or non-stick spray.
Step 2: Combine ingredients. In a large, heavy-bottomed saucepan over medium-low heat, combine the sweetened condensed milk, chocolate chips, and butter.
Step 3: Melt and stir. Continuously stir the mixture with a silicone spatula or wooden spoon as the chocolate and butter melt. Continue stirring until the mixture is completely smooth and all the chocolate is melted and fully incorporated. This usually takes about 5-7 minutes.
Step 4: Remove from heat and add vanilla. Once smooth and glossy, remove the pan from the heat. Stir in the vanilla extract and an optional pinch of salt. If adding any mix-ins like nuts, fold them in now.
Step 5: Pour and spread. Immediately pour the hot fudge mixture into your prepared pan. Use the spatula to spread it evenly into all corners of the pan, working quickly.
Step 6: Chill to set. Place the pan in the refrigerator for at least 2-4 hours, or until the fudge is firm. For best results, chill overnight.
Step 7: Cut and serve. Once thoroughly chilled and firm, use the parchment paper overhang to lift the entire block of fudge out of the pan. Place it on a cutting board and cut it into 1-inch squares using a sharp knife. For clean cuts, wipe the knife blade with a warm, damp cloth between each slice.