In a medium bowl, sift together flour and cocoa powder. Set aside.

In a large bowl, beat butter and sugar with a mixer on medium speed until fluffy. Add egg yolks, one at a time, beating well after each addition. Turn off mixer, and add sifted flour mixture and salt. Mix on low speed until just combined. Do not overmix.

Wrap dough in plastic wrap and refrigerate until chilled, at least 1 hour, before using.