On medium speed beat butter and sugar until creamy, 3-4 minutes, stopping to scrape the bowl. Add egg yolks, one at time, beating well after each addition. In a medium bowl, sift together flour, black cocoa, and salt. With mixer on low-speed, gradually add flour mixture to butter mixture, beating just until combined. (Don’t overbeat.) Turn out dough, and shape into a disk. Wrap in plastic wrap, and refrigerate for at least 1-hour.