In a saucepan, over medium low heat, add sweetened condensed milk. Once hot, but NOT boiling, add sugar cookie mix and stir.
Next, add white chocolate chips, butter and vanilla to the same pot, still over medium low heat.
Heat until totally melted and smooth, continue to stir frequently. If needed, to smooth mixture, add another tablespoon of butter. I always do. This mixture will be thick.
If using, pour in half your sprinkles and only stir 1-2 times so your sprinkles don't melt!. If you don't like sprinkles check below in the recipe notes for some other suggestions!
Line an 8x8 or 9x9 inch square baking dish with parchment paper and spray with nonstick cooking spray.
Top with sprinkles too, if desired.
Pour fudge into pan, and refrigerate for at least 2 hours, if not 4.
Once cooled, cut into squares and serve. Enjoy!