Sift flour and salt into a medium bowl; set aside. Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment, and mix on medium speed until pale and fluffy, 2 to 3 minutes, scraping down sides of bowl as needed. Reduce speed to low. Mix in egg yolk and vanilla. Gradually add flour mixture; mix until just combined, about 1 minute.
Remove half of the dough; set aside. Add cocoa powder to remaining dough; mix on low speed until well combined. Turn out chocolate dough onto a lightly floured work surface. Roll into a 10-inch log, about 1 1/2 inches in diameter. Repeat with reserved vanilla dough. Wrap each log in plastic wrap, and refrigerate until slightly firm, at least 30 minutes.
Press handle of a long wooden spoon into side of chocolate log, making an indentation along its length. Roll handle into and then away from log, creating an apostrophe shape. Repeat with vanilla log. Fit logs together; press lightly to seal. Gently roll into a 2-inch-diameter log. Wrap in plastic wrap, and freeze until firm, about 30 minutes.
Preheat oven to 350 degrees. Cut log into 1/4-inch-thick rounds; space 1 inch apart on baking sheets lined with parchment paper. If dough becomes too soft to slice cleanly, return to freezer until firm.
Bake until firm to the touch, 10 to 12 minutes. Transfer to wire racks; let cool. Cookies can be stored in airtight containers at room temperature up to 3 days.