Preheat oven to 350 degrees.
Sift together cake, baking powder and salt; set aside.
Cream together butter and sugar in mixing bowl until light and fluffy, using electric mixer at medium speed.
Add eggs, one at a time, beating well after each addition.
Blend in vanilla and cooled chocolate, mixing well.
Combine baking soda and iced water; stir until baking soda is dissolved.
Add dry ingredients alternately with iced water mixture to creamed mixture, beating well after each addition.
Pour patter into greased 13 x 9 inch baking pan.
Bake 35 minutes or until cake tests done.
Cool in pan on rack 10 minutes.
Remove from pan; cool on rack.

Whipped Chocolate Frosting:
Combine chocolate and butter or margerine in small saucepan.
Heat over low heat until chocolate and butter are melted.
Cool slightly.
Combine cooled chocolate mixture, powdered sugar, egg, salt, milk, and vanilla in metal mixing bowl.
Set bowl in another bowl filled with ice cubes and water.
Beat with electric mixer at high speed until mixture becomes light and fluffy and forms soft peaks, about 5 minutes.
Frost sides and top of cake.
Cut in squares.