The second, more elaborate, version from this box. The first is here.
From the box of A.D. from Lutz, Florida, by way of Pennsylvania in the 1940s, and originating in Ohio in the 1920s.
Chocolate Layer Cake
1/3 cup butter
1 cup sugar
1 egg
1 tsp. vanilla
1 c. milk
1-3/4 c. flour
4 tsp. baking powder
1/4 tsp. salt
Cream well. Add ingredients and beat to a smooth batter. Bake in layer in moderate oven, 15 to 20 minutes.
Filling:
3 Tbsp. cream
1 egg yolk
2-1/2 squares unsweetened chocolate
3/4 c. powdered sugar
1 Tbsp. cornstarch
1/8 tsp. salt
1 tsp. vanilla
Save egg white for icing.
Melt chocolate, add cream and egg yolk. Mix in sugar. Add cornstarch which has been mixed with a little cold water and cook until smooth and thick. Add salt and vanilla. Spread between layers.
Icing
1 unbeaten egg white
1-1/4 c. sugar
1 tsp. butter
1 Tbsp. cream
1-1/2 square of unsweetened chocolate
Melt chocolate. Put egg white in shallow dish. Add sugar while beating. Add butter, melted chocolate and cream. Beat well until right consistency to spread.