I first buttered my 8×8 pan lightly and lined it with parchment, leaving an overhang on two opposite sides so I could lift the block out later.
In a medium saucepan over medium heat, I added the butter, dark chocolate chips, sweetened condensed milk, vanilla, and salt. I stirred constantly until everything melted together into a smooth, uniform mixture.
Then I poured the melted fudge into the lined pan, smoothing the top with a spatula (I sometimes press the bottom of a measuring cup to flatten). I sprinkled on my festive sprinkles before the fudge set.
I placed the pan in the refrigerator and let it chill for about 2 hours. After that, I ran a knife around the edges, lifted the fudge using the parchment overhang, and cut it into small squares.