Beat egg whites until stiff.
Beat margarine and sugar together until the mixture is fluffy.
Melt chocolate and stir till smooth.
Add egg yolks, one at a time, stirring till smooth. Combine the chocolate mixture with margarine and sugar.
Fold in beaten egg whites.
Place 2/3 of mixture into 10 inch springform pan and bake at least 45 minutes, until knife inserted in center comes out almost clean.
Place remaining 3rd of mousse in refrigerator.
Let cake cool thoroughly; it will sink.
Take out remaining mousse from refrigerator and stir once or twice. Pour into center of cooled cake and spread to within 2 " from edge, or until sides rise.*
Keep in refrigerator until serving (remove side of pan and slice)
*I don't serve anything with raw eggs in it. Instead, I will use two-thirds of the listed amounts to make the cake portion, and make up a separate mousse using pasteurized egg products.