Using an electric mixer on medium-high speed or a large bowl and wooden spoon, beat butter and cream cheese until creamy and smooth. With the machine running, add sugar and salt, and continue beating until a little fluffy. Add egg yolk and beat until incorporated. (Reserve egg white.) Add flour all at once and mix just until incorporated. Halve the dough, and place each half on plastic wrap. Using the plastic wrap, press each half into a 1-inch-thick rectangle. Wrap tightly and refrigerate until firm, about 2 hours.
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