Prepare your pan by lining an 8×8 inch baking dish with parchment paper, leaving overhang on the sides for easy removal. Lightly grease the parchment if desired.
Combine the chocolate chips, condensed milk, and butter in a medium saucepan. Heat over low heat, stirring constantly to prevent scorching.
Continue stirring until completely melted and smooth – this should take about 5 minutes. The mixture should look glossy and uniform with no visible chocolate chip pieces remaining.
Remove from heat and stir in vanilla and salt until fully incorporated. This is the moment that transforms good fudge into great fudge, so don’t skip these flavor enhancers.
Fold in any optional mix-ins if using, distributing them evenly throughout the mixture without overmixing.
Pour the fudge mixture into your prepared pan and smooth the top with your spatula for a perfectly even surface.
Allow to cool completely at room temperature for about 2 hours, then transfer to the refrigerator for at least 1 hour to fully set.
Lift the fudge out using the parchment paper overhang and place on a cutting board.
Cut into 1-inch squares with a sharp knife, wiping the blade clean between cuts for the cleanest edges.