Heat oven to 350°F. Grease 9 x 2-inch round pan; line bottom with waxed paper; grease paper; dust pan with flour.
On low speed, mix flour, sugar, cocoa, baking soda, baking powder and salt in large bowl until mixed.
Add eggs, buttermilk, water, butter and vanilla; beat 30 seconds on low speed, until dry ingredients are moistened. Increase to speed medium; beat 2 minutes. Pour into prepared pan.
Bake in 350°F oven for 40 minutes, or until wooden pick inserted in center comes out clean. Cool cake in pan on rack 10 minutes. Remove cake from pan to rack to cool.
Beat together butter, milk, vanilla, salt and 1 cup of the powdered sugar in medium-size bowl until smooth and creamy. Gradually beat in remaining powdered sugar until good spreading consistency.
When cake is completely cooled, slice off rounded top if necessary to flatten. (Reserve trimmings for snacks.) Invert cake onto large serving platter. Spread frosting over sides and top of cake, using long, thin metal spatula to smooth evenly.