Preheat the oven to 350 degrees. Grease and flour a 13 x 9 x 2-inch pan.
Combine the flour, sugar, cocoa, baking soda and salt in a large mixing bowl.
Add the water, oil, vinegar and vanilla; beat for 3 minutes at medium speed until thoroughly blended.
Pour the batter into the prepared pan and bake at 350 degrees for 35 to 40 minutes or until a cake tester inserted in the middle of the cake comes out clean. Cool and frost the cake in the pan.
Source: “Hershey’s Cocoa Cookbook”, Western Publishing Company, 1979