Preheat oven to 325-350ºF.
Grease & flour three 9-inch round cake pans.
In medium bowl, gradually stir boiling water into cocoa until blended. Set aside to cool completely.
On sheet of waxed paper, combine flour, baking soda, baking powder and salt, set aside. In large bowl, beat butter with sugar at high speed until light and fluffy. Add eggs, one at a time, beating after each. Beat in vanilla. Beat in flour mixture (in fourths) alternately with cocoa mixture (in thirds) beginning and ending with flour and beating each addition just until blended. Pour batter into prepared pans: bake 25 minutes or until cake tester comes out clean. Cool in pans 10 minutes: remove & cool on wire racks.
In top of double boiler placed over hot, not boiling water, combine white chocolate with ¼ cup heavy cream. Heat, stirring, until chocolate melts and is smooth. Remove from heat: pour into large bowl. Stir in remaining heavy cream: refrigerate until very cold. At high speed. beat filling until stiff. Spread between cooled cake layers.
In medium saucepan, over medium heat, combine chocolate pieces, half-and-half and butter. Cook, stirring until smooth. Transfer mixture to large mixing bowl. Gradually beat in confectioners' sugar. (I had trouble getting it thick enough, so I put it in the fridge awhile. Just don't leave it in too long or you'll have to warm it again). Frost top and sides of cake. Enjoy.