Step 1:Prepare the pan: Line an 8x8 inch square baking pan with parchment paper, leaving some overhang on the sides. This will make it easy to lift the fudge out later. Set aside.

Step 2:Combine ingredients: In a medium saucepan, add the chocolate chips, sweetened condensed milk, and butter. Place over LOW heat. This is important - low heat prevents the chocolate from burning.

Step 3:Melt and stir: Stir constantly with a wooden spoon or silicone spatula as the ingredients melt together. Continue stirring for 5-7 minutes until the mixture is completely smooth, glossy, and no chocolate chips are visible.

Step 4:Add extras: Once the mixture is smooth, remove from heat. If using, stir in the vanilla extract and a pinch of sea salt. Mix until well combined.

Step 5:Pour into pan: Pour the fudge mixture into your prepared pan. Use a spatula to spread it evenly into all corners and smooth the top surface.

Step 6:Chill: Place the pan in the refrigerator and chill for at least 2-3 hours, or until the fudge is completely firm to the touch. For best results, chill overnight.

Step 7:Cut and serve: Once set, use the parchment paper overhang to lift the fudge out of the pan. Place on a cutting board. For clean cuts, dip a sharp knife in hot water, wipe it dry, then cut. Clean and warm the knife between each cut. Cut into 36 one-inch squares.

Step 8:Store: Store fudge in an airtight container in the refrigerator for up to 2 weeks, or freeze for up to 2 months. Place parchment paper between layers to prevent sticking.