Heat the oven to 350°F (175°C). Grease and line two or three 8-inch round cake pans with parchment, grease the parchment, and lightly flour the sides.
Toast the pecans on a sheet pan for 7–8 minutes, then cool and chop.
Whisk the flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl until combined.
In a separate bowl, whisk the buttermilk, oil, eggs, and vanilla until smooth, then stir in the melted German chocolate.
Pour the wet ingredients into the dry ingredients and whisk just until no dry spots remain.
Stream in the hot coffee and whisk until the batter is glossy and fluid; the batter will be thin.
Divide the batter evenly among the prepared pans, tap once to release large bubbles, and bake 22–28 minutes until a toothpick comes out with a few moist crumbs.
Cool the cakes in the pans for 10 minutes, then turn out onto racks and cool completely.
For the coconut-pecan filling, whisk the granulated sugar, brown sugar, butter, evaporated milk, egg yolks, and salt in a medium saucepan.
Cook over medium heat, stirring constantly, until thick and bubbling and the custard coats a spatula and leaves a trail when you drag a finger across, about 8–10 minutes.
Remove from heat and stir in vanilla, shredded coconut, and chopped toasted pecans, then spread on a sheet pan to cool until thick and just slightly warm.
For optional ganache, heat the cream until steaming, pour over the chocolate (and corn syrup if using), let sit 2 minutes, then stir until smooth and glossy. Cool to a pourable consistency.
Level the cooled cake layers if needed. Place the first layer on a stand and spread about 1 1/4 cups filling edge to edge.
Add the second layer and spread more filling; for a three-layer cake, add the third layer and spread remaining filling on top.
Leave the sides bare or add a thin coat of ganache, then drizzle ganache over the top, nudging to create slow drips.
Chill the assembled cake for 20–30 minutes to set before slicing.