Cream butter until light and fluffy adding the sugar and yolks gradually. Sift flour, soda and cocoa together.
Add the flour-cocoa-soda mixture alternately with the carnation evaporated milk beginning and ending with the flour mixture. Beat egg whites adding the sugar gradually until stiff but not dry.
Fold the egg whites into the chocolate mixture using the cut and fold strokes.
Pour into prepared pans and bake in moderate oven (350°F) for 40-45 minutes or until cake springs back when lightly touched. Cook the cake