Preheat your oven to 350°F (175°C). Grease three 8-inch round cake pans with butter or nonstick spray. Line the bottoms with parchment rounds and lightly flour the sides.

Place the chopped sweet baking German chocolate in a heatproof bowl. Pour the boiling water over the chocolate and let sit for 1–2 minutes, then stir until smooth and glossy. Set aside to cool until warm but not hot.

In a large mixing bowl, beat the softened butter and 2 cups of granulated sugar with a hand mixer or stand mixer on medium-high speed for 3–4 minutes, until pale and fluffy. Add the 4 egg yolks one at a time, beating well after each addition. Mix in 2 teaspoons vanilla extract and the melted German chocolate until fully combined.

In a separate bowl, whisk together the cake flour, baking soda, and 1/2 teaspoon fine sea salt. With the mixer on low, add the dry ingredients in three parts, alternating with the buttermilk (beginning and ending with the flour): flour, buttermilk, flour, buttermilk, flour. Mix just until no dry flour pockets remain.

In a clean bowl, beat the 4 egg whites on medium-high speed until stiff peaks form. Gently fold the beaten egg whites into the batter in two additions using a spatula, taking care not to deflate them.

Divide the batter evenly among the three prepared pans. Smooth the tops and bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Cool the cakes in the pans on a wire rack for 10–15 minutes, then run a knife around the edges, invert onto racks, remove parchment, and cool completely.

In a medium saucepan, whisk together the evaporated milk, 1 cup granulated sugar, and 3 egg yolks until smooth. Add 1/2 cup unsalted butter pieces and cook over medium heat, stirring constantly, for 8–12 minutes, until thickened and pudding-like and the mixture coats the back of a spoon. Remove from heat and stir in 1 teaspoon vanilla extract, shredded coconut, and chopped pecans. Transfer to a bowl and cool to room temperature, stirring occasionally.

For frosted sides or an extra chocolate layer, beat 1/2 cup softened butter in a mixing bowl until creamy. Add powdered sugar, cocoa powder, and 1/4 teaspoon fine sea salt and mix on low until combined. Add 3 tablespoons cream or milk and 1 teaspoon vanilla, then beat on medium-high for 2–3 minutes until light and fluffy. Adjust consistency with a little more cream or powdered sugar as needed.

Place one cooled cake layer on a serving plate or cake stand. If using chocolate frosting, spread a thin layer over the top. Spoon about 3/4 to 1 cup of coconut pecan frosting over the layer and spread almost to the edges. Repeat with the second layer: cake, optional chocolate frosting, coconut pecan frosting. Place the final cake layer on top. Frost the sides with remaining chocolate frosting, if using, then pile the remaining coconut pecan frosting on top, spreading to the edges and swirling decoratively.

Serve at room temperature. The cake can sit lightly covered at room temperature for up to 24 hours. After that, refrigerate covered for up to 4–5 days, bringing slices to room temperature for 20–30 minutes before serving.