Preheat oven to 350 degrees.
Grease and flour 3 9 inch round cake pans.
Combine cocoa and boiling water stirring until smooth; set aside.
Cream butter, gradually add sugar, beating well at medium.
Add eggs, one at a time, beating well after each addition.
Combine flour, soda, baking powder, and salt.
Add to creamed mixture alternating with cocoa mixture, beating at low speed, beginning and ending with the flour mixture.
Stir in vanilla.
Do not overbeat.
Pour into pans and bake 20 minutes or until a wooden pick inserted in center comes out clean.
Cool pans 10 minutes.
Remove the cake from pans; cool completely.

Filling:
Beat whipping cream and vanilla until foamy.
Gradually add powdered sugar beating until soft peaks form.
Chill.
Spread filling between layers.

Frosting:
Combine chocolate, cream, and butter in a heavy saucepan.
Cook over medium heat, stirring until chocolate melts. Remove from heat.
Add powdered sugar; mix well.
Set saucepan in ice and beat at low speed until frosting holds its shape and loses its gloss.
Add a few more drops of cream if needed to make spreadable.
Spread frosting on top and sides.
Chill until serving time.