Step 1 Heat the oven to 350 degrees. Butter and flour a 9-inch round cake pan.
Step 2 Beat together the butter and sugar until light and creamy, about 4 minutes. Stir the cocoa and water together in a bowl and add to the butter mixture. Beat 7 minutes, scraping down the sides of the bowl often. Add the flour, salt, baking powder and baking soda. Beat 2 minutes longer. Add the eggs. Mix until the batter is shiny and smooth, 5 minutes, scraping down the sides of the bowl.
Step 3Pour the batter into the pan. Bake the cake until the center springs back when lightly touched, about 40 minutes. Invert the cake onto a wire rack to cool completely.