Line a 9x9 inch square pan with foil or parchment paper. If using parchment paper, clip the paper over the sides of the pan with binder clips to help keep the parchment paper in place. Lightly spray the foil/parchment paper with nonstick cooking spray.
In a medium saucepan, add in the sweetened condensed milk.
Place over medium heat until the milk is hot and small bubbles form around the edge of the saucepan, stirring often.
Add the white chocolate, sugar cookie mix, butter, and vanilla extract into the saucepan while it’s still on the heat, stirring constantly until the chocolate and butter has melted and the fudge mixture is smooth.
Once the fudge is completely smooth, pour into the prepared pan.
Sprinkle the top of the fudge with Christmas sprinkles or sanding sugar and place in the fridge for at least 3 hours to allow the fudge to fully set.
Once set, remove from the fridge.
Remove the fudge from the pan and cut into 36 pieces.
Store in an airtight container at room temperature for up to 5 days or in the refrigerator for 10 days.