TIMING NOTES: Before we start, let's talk timing...you don't want to frost the cake when it's super hot, AND the frosting needs to set for 25 minutes once it's done, before putting on the cake. So your best bet is to pull the cake out of the oven and then make the frosting, let it sit for ~25 minutes, and it should be perfect. Or you can bake the cake a bit ahead of time and then it will be totally cool when you frost it.

Make the cake: Preheat the oven to 350°F. Spray a 9x13 pan and set aside.

In a medium microwave-safe bowl, combine the German chocolate (broken into pieces) and hot water, and microwave in 30-second intervals, stirring between each, until the chocolate is completely melted and smooth. Set aside.

In the bowl of your stand mixer (fitted with whisk attachment), mix the buttermilk, sour cream, and oil until combined. Then whisk in the eggs and vanilla.

Add the flour, cocoa, baking powder, baking soda, Postum or espresso powder, sugar, and salt, along with the melted chocolate mixture. Gently mix until everything is incorporated. It will be a very liquid batter.

Pour the batter into your prepared pan and (carefully) bang the pan on the counter a few times to release any air bubbles. Bake for 30-40 minutes, until a toothpick inserted in the center comes out with a few moist crumbs. Let cool mostly or entirely before frosting.

Make the frosting: Spread the coconut and pecans on two ends of an extra-large baking sheet (or a large one and a half-sheet, separately). You want to keep the pecans separate, as you'll need to chop them later. Bake for 5-7 minutes, stirring every couple of minutes, until toasted. Once cool enough to handle, chop the pecans very fine.

In a large saucepan, add the butter, brown sugar, egg yolks, evaporated milk, and fine salt.

Cook over medium heat, whisking constantly, as the mixture comes to a low boil. Once boiling, whisk constantly for 5 minutes (I suggest setting a timer), until thickened. It should be thickened to a custard-like consistency...thick but pourable.

Remove from heat and stir in the vanilla and large-crystal sea salt, then add the chopped pecans and toasted coconut. Stir everything together and let it cool completely before using, around 2 minutes. If it doesn't seem quite thick enough once it's cooled, you can stir in a bit more coconut.

Frost the cake. It's best served at room temp, though you should store it in the fridge if you'll be keeping it more than a day or two.