Line a 9x9-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal later.
In a medium saucepan over low heat, combine the chocolate chips and sweetened condensed milk. Stir continuously until the chocolate is completely melted and the mixture is smooth. Alternatively, melt in a microwave-safe bowl in 30-second increments, stirring in between.
Remove from heat and stir in kosher salt, vanilla extract, and espresso powder (if using) until well combined.
Fold in any optional mix-ins, gently ensuring they are evenly distributed.
Pour the fudge mixture into the prepared pan, spreading it evenly with a spatula. Tap the pan lightly to release air bubbles.
Refrigerate the fudge for at least 2 hours or until firm enough to cut.
Lift the fudge out using the parchment overhang, cut into squares, and serve. Store leftovers in an airtight container in the refrigerator.