The recipe is a bit complicated, but worth the trouble. Taken from the Fannie Farmer Cookbook.

Batter
Ingredients
6 Tbsp butter
1 c sugar
2 eggs, separated
1 1/2 tsp vanilla
1 3/4 cups cake flour
2 tsp baking powder
1/2 tsp salt
2/3 c milk

Cooking
1) Preheat oven to 350F, butter and lightly flour two 8-inch round pans.
2) Microwave butter on low until a little soft, then beat (cream).
3) Slowly add the sugar, beating until light.
4) Add yolks and vanilla, beat to blend well.
5) Sift flour, baking powder, and salt into a bowl, mix together.
6) Alternate blending dry ingredients and milk in, in 3 stages. Beat until smooth.
7) Separately, beat the egg whites until stiff. Stir 1/3 in, fold the rest in gently.
8) Bake 30-35 minutes, then cool.

Filling
Ingredients
1 c milk
1/2 c sugar
3 tbsp flour
1/8 tsp salt
2 egg yolks
1 1/2 tsp vanilla

Cooking
1) Assemble the ingredients.
2) Heat milk in a pan until very hot, stirring constantly.
3) Add sugar, flour, salt very quickly.
4) Cook until very thick (~5 minutes), stirring constantly.
5) Add the egg yolks, cook another ~5 minutes, stirring constantly.
6) Remove from heat, add vanilla, and cool, stirring occasionally.
7) Cover and refrigerate.

Combining
Ingredients
4 1/2 oz semisweet chocolate (Ghirardelli is good)
3 Tbsp butter

Assembly
1) Spread filling between the two pieces of cake.
2) Microwave chocolate and butter together in the microwave on low until butter melts.
3) Blend the chocolate/butter mixture and cover the cake with it.
4) Refrigerate.