Cream butter and sugar, then mix in vanilla, milk salt, and flour using a hand or a stand mixer. Mix until a cookie dough forms.
Melt white chocolate in the microwave on 50% power in 30 second increments, stirring between each, until melted and smooth. (Or follow the package directions.)
Line a cookie sheet with wax or parchment paper. Spread half the chocolate into an approximately 8x10 rectangle on the cookie sheet.
Scoop handfuls of the cookie dough and flatten them between the palms of your hand, then place onto the melted chocolate. Cover the entire rectangle, leaving a small border around the edges. Spread remaining white chocolate over the top and sprinkle with sprinkles, as desired.
Chill until firm. Use a knife to cut into pieces. Store in the refrigerator, serve room temperature.