1Whisk the yolks in a medium saucepan off the stove. Gradually whisk in the evaporated milk. Add the sugars, butter, and salt and cook over medium-high heat, whisking constantly, until the mixture is boiling, frothy, and slightly thickened, about 6 minutes.
Pour the mixture into a bowl, whisk in the vanilla, then stir in the coconut. Let cool until room temperature. Cover with plastic wrap and refrigerate until cool, at least 2 hours or up to 3 days. (Do not add the pecans now or they’ll turn soggy. Have a little patience.).
Preheat the oven to 350°F (175°C). Toast the pecans on a rimmed baking sheet until fragrant and browned, about 8 minutes. Transfer to a plate to cool.
Make the German chocolate cake:
Keep your oven at 350°F (175°C) and adjust an oven rack to the lower-middle position. Combine the chocolate and cocoa in a small bowl and then add the boiling water over. Let stand to melt the chocolate, about 2 minutes. Whisk until smooth and let stand until room temperature.
Meanwhile, spray two 9-inch-round by 2-inch-high straight-sided cake pans with nonstick cooking spray and then line the bottoms with parchment or waxed paper rounds cut to fit. Spray the paper rounds, dust the pans with flour, and tap out any excess flour. Sift the flour and baking soda into a medium bowl or onto a sheet of parchment or waxed paper.
(Liner instructions: Tear off a piece of wax paper a couple of inches larger than your cake pan. Put your cake pan on the wax paper and with your scissors or a knife "draw" a line around the base of your cake pan. Next, cut your wax paper just slightly inside the line just drawn. This will make the wax paper liner slightly smaller than the base of your cake pan.).
In the bowl of a standing mixer, beat the butter, sugars, and salt at medium-low speed until the sugar is moistened, about 30 seconds. Increase the speed to medium-high and beat until the mixture is light and fluffy, about 4 minutes, scraping down the bowl with a rubber spatula halfway through. With the mixer running at medium speed, add the eggs one at a time, beating well after each addition and scraping down the bowl halfway through. Beat in the vanilla, then increase the speed to medium-high and beat until light and fluffy, about 45 seconds. With the mixer running at low speed, add the chocolate mixture, then increase the speed to medium and beat until combined, about 30 seconds, scraping down the bowl once. (The batter may appear broken; this is okay.) With the mixer running at low speed, add the dry ingredients in 3 additions, alternating with the sour cream in 2 additions, beginning and ending with the dry ingredients. Beat in each addition until barely combined. After adding the final flour addition, beat on low until just combined, then stir the batter by hand with a rubber spatula, scraping the bottom and sides of the bowl. The batter will be thick. Divide the batter evenly between the prepared cake pans, spreading the batter to the edges of the pans with the rubber spatula and smoothing the surface.
Bake the cakes until a toothpick inserted into the center of the cakes comes out clean, about 30 minutes. Cool in the pans 10 minutes, then invert the cakes onto a greased wire rack; peel off and discard the paper rounds. Cool the cakes to room temperature before filling, about 1 hour. (The cooled cakes can be wrapped in plastic wrap and stored at room temperature for up to 1 day.).
Assemble the German chocolate cake:
Stir the toasted pecans into the chilled filling. Set 1 cake layer on a serving platter or cardboard round that’s cut slightly smaller than the cake. Place the second cake on a work surface or leave it on the wire rack. Hold a serrated knife held so the blade is parallel with the work surface and use a sawing motion to cut each cake into 2 equal layers. Carefully lift the top layer off each cake.
Using an icing spatula, distribute about 1 cup filling evenly on the cake layer on the serving platter or cardboard round, spreading the filling to the very edge of the cake and evening the surface. Carefully place the upper cake layer on top of the filling. Repeat using the remaining filling and cake layers. Dust any crumbs from the platter and serve. (The cake may be refrigerated, covered loosely with foil, up to 4 hours. If the cake has been refrigerated longer than 2 hours, let it stand at room temperature for 15 to 20 minutes before slicing and serving.).