Gold from egg yolks. Silver cake is another name for white cake, and “plain cake” meant a yellow cake made with whole eggs, but gold cake is always gold cake.
This recipe from the Dec 16, 1871 edition of the Janesville (Wisconsin) Gazette shows the gold and silver distinction:
Silver Cake and Gold Cake
One cup white sugar, half cup butter, whites of five well-beaten eggs, quarter teaspoonful soda dissolved in half teacupful milk, three quarters teaspoonful cream tartar mixed with two cups of flour. Flavor with extract of bitter almonds. The yolks of these five eggs, and the same ingredients, make gold cake.
From the box of A.D. from Lutz, Florida, by way of Pennsylvania in the 1940s, and originating in Ohio in the 1920s.
5 Tbsp. Crisco
3/4 c. sugar
1/2 c. milk
4 yolks of eggs
1-1/2 c. flour
1/2 tsp. salt
3 tsp. baking powder
1/2 tsp. lemon extract
Cream Crisco and sugar. Beat egg yolks very light and add to creamed mixture.
Add dry ingredients, milk and lemon and mix well.
Bake in moderate oven 45 minutes.