In medium bowl, combine cocoa with boiling water, mixing with wire whisk until smooth.
Sift flour with soda, salt and baking powder.
Preheat oven to 350F.
Grease well and lightly flour three 9 by 1 1/2 inch layer-cake pans.
In large bowl of electric mixer, at high speed, beat butter, sugar, eggs, and vanilla, scraping bowl occasionally until light-about 5 mins.
At low speed, beat in flour mixture (in fourths), alternately with cocoa mixture (in thirds), beginning.
and ending with flour mixture.
Do not overbeat.
Divide evenly into pans;smooth top. Bake 25 to 30 mins.
, or until surface springs back when gently pressed with fingertip.
Cool in pans 10 mins.
Carefully loosen sides with spatula; 4. Remove from pans; cool on racks.
Frosting:In medium saucepan, combine chocolate pieces,cream, butter;stir over medium heat until smooth.
Remove from heat.
With whisk, blend in 2 1/2 cups confectioners' sugar.
In bowl set over ice(I use ice cubes) beat until it holds shape.
Filling: whip cream with sugar and vanilla;refrigerate.
To assemble cake: On plate place a layer, top side down;spread with half of cream.
Place second layer, top side down;spread with rest of cream.
Place third layer, top side up. To frost: With spatula, frost sides first,covering whipped cream;use rest of frosting on top, swirling decoratively.
Refrigerate at least 1 hour before serving.