01 - Line an 8-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a medium saucepan over low heat, combine the semi-sweet chocolate chips, sweetened condensed milk, and butter pieces. Stir constantly until fully melted and smooth, about 4 to 5 minutes.
03 - Remove the saucepan from heat and stir in the vanilla extract until evenly incorporated.
04 - Immediately pour the fudge mixture into the prepared baking pan, using a spatula to smooth the top into an even layer.
05 - Scatter the holiday sprinkles evenly across the surface, then gently press them into the fudge so they adhere.
06 - Refrigerate the fudge for at least 2 hours, or until it is completely firm to the touch.
07 - Lift the fudge slab out of the pan using the parchment overhang. Place on a cutting board and slice into 25 even squares. Serve chilled or at room temperature.