Line an 8×8 pan with parchment paper, set aside.
In a medium sized bowl, cream together the butter, sugar, milk and vanilla until thoroughly combined.
Mix in the flour and salt until completely incorporated. Set aside.
In a small glass bowl, microwave the white chocolate and sweetened condensed milk until smooth. Microwave for 1 minute and 30 seconds, stirring every 30 seconds.
Pour the white chocolate mixture into the sugar cookie dough bowl. Combine until smooth.
Pour the fudge into the prepared pan, spread evenly.
Add sprinkles to the top of fudge, if desired.
Refrigerate for 2 hours before cutting.
Store in the refrigerator until ready to serve.