Traditional German chocolate cake relies on a rich filling and frosting made of egg yolks, sugar, evaporated milk and butter to bind and decorate its chocolate layers. This frosting, which is studded with coconut and pecans, needs to be thick enough to hold the layers together. If you've had trouble achieving the proper thickness in the past, plan ahead and make the frosting at least several hours before the cake. Making the frosting first gives you time to correct a runny frosting while it is hot, as well as to chill it so it thickens fully.
Combine the basic frosting ingredients and gradually bring them to a boil in the saucepan.
Wait for the frosting to turn golden, which takes 3 to 4 minutes. If the mixture is not thick enough, boil and stir the frosting another 1 to 2 minutes.
Remove the frosting from the heat and stir in the coconut. The coconut may add enough structure to the frosting to alleviate any worries about consistency.
Add a small amount of cornstarch or confectioner's sugar if the mixture remains runny. Whisk it in while the frosting is still hot, adding small pinches until the frosting begins to thicken. Too much can lead to graininess or an off taste, especially with cornstarch. Keep the total quantity to 1 tablespoon or less.
Turn the frosting into a mixing bowl and let the mixture cool completely. If it still seems too thin for spreading, put it in the refrigerator to stiffen it. The filling can be refrigerated for up to 3 days.
Fold the toasted pecans into the cold frosting, and assemble the cake. Traditionally, German chocolate cake is made by putting the coconut-pecan frosting-filling between the layers, and covering the top. The sides are left unfrosted.