Mix cocoa, egg yolk, and 1 cup (250 mL) of the skim milk in a saucepan.
Cook over low heat, stirring, until smooth and thickened.
Cream margarine until light and fluffy.
Create sweetener by combining sorbitol and sugar replacement in a bowl, stirring to mix.
Gradually add sweetener to creamed margarine, beating constantly.
Continue beating until light and fluffy.
Add vanilla and eggs, one at a time, beating well after each addition.
Beat in cocoa mixture.
Sift dry ingredients together.
Add dry ingredients and remaining milk alternately to the creamed mixture. (Begin and end with dry ingredients.) Beat well after each addition.
Continue beating until smooth.
Pour batter into two 9 in. (23 cm) round layer pans.
Pans should be greased and their bottoms lined with greased waxed paper.
Bake at 350°F (175°C) for 25 to 30 minutes or until done.
Cool in pans for 5 minutes.
Turn out onto rack, remove paper, and cool completely.