Preheat oven to 325 degrees. Butter two 5" x 9" loaf pans; set aside.
Combine all-purpose flour, unsweetened cocoa powder, and salt in a bowl.
Cream butter and sugar with a mixer on high speed until pale and fluffy, for 8 minutes. Scrape sides of bowl. Reduce speed to medium and add vanilla extract.
Lightly beat eggs, and add to mixer bowl in four additions, mixing thoroughly after each and scraping down sides. Reduce speed to low, and add flour mixture in four additions, mixing until just incorporated. Divide batter between pans. Tap on counter to distribute; smooth tops.
Bake until a tester inserted into center of each cake comes out clean, about 65 minutes. Let cool in pans on a wire rack for 30 minutes. Remove from pans, and let cool completely on wire rack.