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. Stir constantly until completely melted and smooth, about 5 minutes.
03 - Remove the saucepan from heat and stir in the vanilla extract until fully incorporated.
04 - Immediately pour the fudge mixture into the prepared baking pan, smoothing the top evenly with a spatula.
05 - Scatter the holiday sprinkles evenly across the surface, gently pressing them into the fudge so they adhere.
06 - Refrigerate for at least 2 hours, or until the fudge 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.