Heat oven to 350°F. Grease and flour three 8-inch round cake pans. Mix flour, baking powder and salt in bowl. Melt chocolate in cream in saucepan over medium heat; let cool.
Beat butter, sugar in large bowl until smooth, 3 minutes. Gradually add egg whites, cooled chocolate. On low speed, beat in flour mix. Divide among pans, 2 cups per pan.
Bake in 350°F oven 25 to 30 minutes, until pick inserted in centers tests clean. Cool in pans on rack 5 minutes; remove from pans; cool. Slice each layer in half horizontally.
Beat butter, sugar, cocoa and milk in bowl. Beat in chocolate and vanilla.
Place 1 cake layer on stand. Spread with scant 1/3 cup frosting. Continue stacking layers. Frost side, top of cake.