Preheat the oven to 350°F. Lightly grease and flour two 9" round cake pans, three 8" round pans, or a 9" x 13" pan., In a large mixing bowl, beat together the butter, sugar, and salt until fluffy and light, beating for at least 5 minutes., In a separate bowl, whisk together the flour, cocoa, and baking powder. If lumps remain, sift the mixture., Add the eggs to the butter mixture one at a time, beating well after each addition., Mix together the milk or water and the vanilla., Add one-third of the flour mixture to the batter, then add half the milk, another third of the flour, the remaining milk, and the remaining flour, stirring to combine after each addition. Scrape the sides and bottom of the bowl occasionally throughout this process. Divide the batter evenly between the prepared pans., Bake the cake(s) for 30 to 35 minutes (a bit longer for the 9" x 13" cake), until a cake tester or toothpick inserted into the center comes out clean, and the cake begins to pull away from the sides of the pan., Remove the cake(s) from the oven, cool them for 5 to 10 minutes, then remove them from their pans (leave the 9" x 13" cake in the pan, for easiest serving). Cool completely before frosting.