In a large saucepan, whisk together egg yolks, evaporated milk, and vanilla extract until smooth.

Add sugar and cubed butter. Cook over medium heat for 12 to 15 minutes, stirring constantly, until thickened like pudding.

Remove from heat; stir in shredded coconut, toasted pecans, and a pinch of salt.

Pour into a heatproof bowl and refrigerate until completely cooled.

Preheat oven to 350°F (176°C). Line the bottoms of three 9-inch cake pans with parchment paper and grease the sides.

Heat water in the microwave until boiling. Pour over chopped German chocolate; let sit 2-3 minutes, then stir until smooth. If needed, microwave an additional 10-15 seconds and stir. Set aside.

In a medium bowl, whisk together flour, baking soda, and salt. Set aside.

In a large mixing bowl, cream butter and sugar until light and fluffy, about 2-3 minutes.

Add egg yolks one at a time, mixing well after each. Mix in vanilla extract and sour cream until combined.

Add the melted chocolate mixture and mix until smooth, scraping down the bowl as needed.

Add half the dry ingredients; mix until combined. Add buttermilk; mix. Add remaining dry ingredients; mix just until combined. Scrape bowl as needed.

In a separate large bowl, whip egg whites on high speed until stiff peaks form.

Gently fold half the whipped egg whites into the batter until mostly combined, then fold in the remaining whites until just combined.

Divide batter evenly among prepared pans. Bake 20-25 minutes, until a toothpick inserted in the center comes out clean.

Cool cakes in pans for 2-3 minutes, then transfer to a wire rack to cool completely.

Once cake layers and filling are completely cool, make the frosting. In a large mixer bowl, beat butter until smooth.

Add cocoa powder and half the powdered sugar; mix until smooth.

Add melted chocolate in 2-3 batches, mixing well after each.

Add salt and 3 tablespoons heavy cream; mix until smooth.

Add remaining powdered sugar; mix until smooth. Add remaining cream as needed for desired consistency.

Use a large serrated knife to level the cake domes.

Place first layer on a serving plate. Pipe a dam of chocolate frosting around the edge, then spread about 1/3 of the coconut pecan filling inside.

Add second layer; repeat dam and filling.

Add third layer; frost the entire cake with remaining chocolate frosting.

Pipe a shell border around the top outer edge (using Ateco tip 847 or similar). Fill the border with remaining coconut pecan filling. Decorate bottom with pecans if desired.

Refrigerate until ready to serve. Best served cool but not cold. Store airtight; best within 4-5 days.