Line an 8x8-inch square baking pan with parchment paper, leaving a 2-inch overhang on two opposite sides for easy removal later.
Combine the chocolate chips, sweetened condensed milk, and butter in a medium saucepan over low heat. Stir constantly until completely melted and smooth, about 5-7 minutes.
Remove the pan from heat and immediately stir in the vanilla extract and salt until evenly incorporated.
Let the mixture cool for 3-4 minutes, stirring occasionally to release steam and prevent separation.
Pour the chocolate mixture into the prepared pan and spread it into an even layer using a rubber spatula.
If using, sprinkle the mini chocolate chips evenly over the top and gently press them into the surface.
Refrigerate uncovered for at least 2 hours, or until completely firm and set throughout.
Use the parchment overhang to lift the fudge slab out of the pan, then cut into 16 squares using a sharp knife. Wipe the knife clean between cuts for the neatest edges.
