Dissolve chocolate in warm water. Let cool.
Cream together butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition.
Sift together flour, baking powder, and salt.
Add flour mixture alternately with orange juice to creamed mixture.
Fold in grated orange peel.
Beat egg whites until stiff peaks form.
Fold egg whites into batter.
Pour into greased and floured 9x12 inch pan.
Bake at 350°F for 25-30 minutes until golden brown.
Cool and cut into squares.
Roll in powdered sugar or orange-flavored glaze if desired.