Preheat oven to 325°F (163°C). Grease and flour a bundt or pound cake pan thoroughly.
In a large bowl, beat butter and sugar until pale and fluffy, about 3 minutes.
Add eggs one at a time, then mix in vanilla and sour cream until smooth.
In another bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt.
Gradually mix into wet ingredients just until combined. Stir in melted German chocolate until glossy.
Pour batter into the pan and bake for 60–70 minutes, until a toothpick comes out clean. Cool 10 minutes, then transfer to a rack.
For the topping, cook evaporated milk, butter, brown sugar, and egg yolks over medium heat, stirring until thickened, 5–7 minutes.
Remove from heat, stir in vanilla, coconut, and pecans. Let cool slightly, then spoon over the cake and let it drip naturally down the sides before serving.