To make the cake, preheat oven to 350F. Grease a 9x13-inch metal baking pan. Set aside.
In a large, heat-proof bowl, add chopped chocolate.
Pour the boiling water over the chocolate, and let it stand for 5 minutes.
Stir until chocolate has melted and is combined with the water.
Add the oil, buttermilk, and sugar. Whisk until well-combined.
Whisk in the eggs and vanilla until well-combined.
Add the flour, cocoa powder, baking powder, baking soda, and salt. Stir in just until combined. Don't overmix.
Pour the batter into the prepared baking pan.
Bake for 25-35 minutes, or until a wooden toothpick inserted in the center comes out clean.
Cool to room temperature.
To make the topping, add the egg yolks, milk, and brown sugar to a large saucepan.
Whisk until combined.
Add butter.
Heat saucepan over low heat.
Cook for 10 minutes, whisking regularly, until mixture has thickened.
Remove from heat.
Whisk in vanilla extract.
Add coconut and pecans, and stir in.
Let topping cool for 15 minutes.
Spread topping over cake, and let cool to room temperature before serving.
Video
Notes
German chocolate: I like to use Baker's German sweet chocolate.
Oil: I've used vegetable oil and melted coconut oil in this cake. If you use melted coconut oil, the buttermilk and eggs should be at room temperature so that the oil doesn't solidify.
Buttermilk: Regular or low-fat will work. If you don't have buttermilk, you can make a substitute with this recipe.