Make the Cookies
In a medium bowl, using an electric mixer at medium speed, beat the butter with the granulated sugar, brown sugar, salt and vanilla until smooth. Add the flour and cocoa and beat at low speed just until incorporated, then beat in the cream until smooth. Turn the dough out onto a work surface and roll it into two 16- inch-long ropes. Wrap the dough in plastic and refrigerate until firm, about 20 minutes.