Place the chopped chocolate and sifted cocoa in a small bowl. Pour the boiling water over them and let it stand for 2 minutes before whisking. This creates a smooth, concentrated "chocolate mud" that distributes evenly through the batter.
Before assembling, toast the pecans at 350°F for 8 minutes until fragrant. Do not add them to the filling until the very last second—this keeps the pecans crunchy rather than softening in the custard.
When mixing, alternate the dry ingredients with the sour cream. This "folding" method prevents the flour from over-developing, ensuring the four thin layers remain light and airy.
Bake in two 9-inch pans. Once cooled, use a long serrated knife held perfectly parallel to the counter. Use a slow sawing motion to split each cake into two even layers, creating four layers total.
Whisk yolks and evaporated milk in a saucepan. Cook with sugars and butter until thickened and frothy. Stir in coconut and vanilla, then refrigerate for at least 2 hours. Cold filling is the secret to a stable four-layer stack.
Place the first layer on a plate. Use an icing spatula to spread 1 cup of the chilled filling all the way to the very edge. Repeat for all four layers.
Leave the sides of the cake exposed to showcase the distinct chocolate and coconut-pecan layers.