Step 1

Preheat oven to 325 degrees.

Step 2

Butter two 5-by-9-inch loaf pans. Combine all-purpose flour and salt in a bowl.

Step 3

Cream butter and sugar with a mixer on high speed until pale and fluffy, for 8 minutes. Scrape down sides of bowl. Reduce speed to medium, and add vanilla extract.

Step 4

Lightly beat eggs, and add to mixer bowl in 4 additions, mixing thoroughly after each and scraping down sides. Reduce speed to low, and add flour mixture in 4 additions, mixing until just incorporated.

Step 5

Butter two 5-by-9-inch loaf pans. Combine all-purpose flour, unsweetened cocoa powder, and salt in a bowl.

Step 6

Cream butter and sugar with a mixer on high speed until pale and fluffy, for 8 minutes. Scrape down sides of bowl. Reduce speed to medium, and add vanilla extract.

Step 7

Lightly beat eggs, and add to mixer bowl in 4 additions, mixing thoroughly after each and scraping down sides. Reduce speed to low, and add flour mixture in 4 additions, mixing until just incorporated.

Step 8

Scoop batters into prepared pans, 1/2 cup at a time, alternating plain and chocolate. Swirl with a knife. Tap on counter to distribute; smooth tops.

Step 9

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.