Marble Cake
from Betty Crocker's Picture Cookbook, 1950s edition

Follow the recipe for Rich White Cake. Pour 2/3 of batter into pans. Beat into remaining batter a mixture of 1 1/2 sq. chocolate (1 1/4 oz.), melted, 1/4 tsp. baking soda, and 2 1/2 tbsp. warm water for larger cake; 1 sq. chocolate, 1/8 tsp. baking soda, and 2 tbsp. warm water for smaller cake. Pour here and there over white batter. Cut through batter with a knife several times for marble effect. Bake. Cool. Frost with chocolate icing.