In a medium bowl, combine cocoa with boiling water, mix with whisk until smooth. Cool completely. Sift flour with soda, salt, and baking powder.
Preheat oven to 350 degrees. Grease well and flour three 9 inch layer cake pans.
In a large bowl mix beat butter, sugar, eggs, and vanilla at high speed until light--about 5 minutes. At low speed, beat in flour mixture (in fourths), alternating with cocoa mixture (in thirds), beginning and ending with flour mixture.
Do not overbeat. Divide evenly into pans. Bake 25 to 30 minutes or until surface springs back when gently pressed with fingertip.
Cool in pans 10 minutes. Then loosen sides with spatula; remove from pans; cool on racks.