In the bowl of a stand mixer fitted with the paddle attachment, beat butter, sugar, and salt on medium speed until creamy, 2 to 3 minutes, stopping to scrape sides of bowl. Beat in egg yolk, milk, and vanilla until well combined.

In a medium bowl, whisk together flour, milk powder, and baking powder. With mixer on low speed, gradually add flour mixture to butter mixture, beating just until combined. Turn out onto a sheet of plastic wrap, and shape into a 10×1½-inch log. Wrap in plastic wrap and refrigerate for at least 1 hour.