IN THE TOP OF A DOUBLE BOILER, MELT CHOCOLATE IN SYRUP, STIRRING OCCASIONALLY. COOL TO LUKE WARM. IN A LARGE BOWL CREAM BUTTER, MARGARINE, AND SUGAR UNTIL LIGHT AND FLUFFY. ADD EGGS, 1 AT A TIME, BEATING WELL AFTER EACH ADDITION. STIR IN VANILLA. SIFT TOGETHER FLOUR AND BAKING SODA, THEN ADD ALTERNATELY WITH BUTTERMILK TO CREAMED MIXTURE. BEAT WELL AFTER EACH ADDITION. WHIP CHOCOLATE MIXTURE WITH LARGE SPOON UNTIL SMOOTH. ADD CHOCOLATE TO CREAMED MIXTURE. POUR BATTER INTO GREASED AND FLOURED 25 cm ANGEL FOOD CAKE PAN. LET SIT FOR ONE HOUR AT ROOM TEMPERATURE FOR BETTER TEXTURE. BAKE AT 350 DEG F. FOR 1 HOUR. TEST WITH TOOTHPICK TO SEE IF DONE. COOL COMPLETELY. TURN OUT ONTO PLATE. SPRINKLE WITH POWDERED SUGAR.