Finely chop chocolate and place into a bowl. Bring cream just barely to a simmer and pour over chopped chocolate. Cover with plastic wrap and let stand 2 mins. Stir with a spatula until combined, smooth, and silky. Set aside to cool completely until thickened but still pourable. See notes.*
Preheat oven to 350F, grease three 6" round baking pans and dust with cocoa powder. Line bottoms with parchment.
Place all dry ingredients into the bowl of a stand mixer fitted with a paddle attachment. Stir to combine.
In a medium bowl whisk all wet ingredients (pour water in slowly as not to cook the eggs if very hot).
Add wet ingredients to dry and mix on medium for 2-3 mins. Batter will be very thin.
Pour evenly into prepared pans. I used a kitchen scale to ensure the batter is evenly distributed. Bake until a cake tester comes out mostly clean. A total of 35-40mins.
Cool 10 minutes in the pans then turn out onto a wire rack to cool completely.
Chocolate Coconut Crumble:
Preheat oven to 350F and line a baking sheet with parchment. Whisk all dry ingredients in a medium bowl. Pour in melted butter and stir until it begins to clump.
Spread evenly on baking sheet and bake for 15mins. Cool completely before using on cake.**
Vanilla Custard Frosting:
Prepare a stand mixer with a whisk attachment. Whisk butter until creamy, scraping bowl as needed.
Reduce speed to low and add in powdered sugar 1 cup at a time until well blended, add custard powder. Increase speed to med and beat for 3 minutes.
Add vanilla and 2 Tbsp cream and continue to whip on medium for 1 minute.
Add more cream as needed until desired consistency is reached (I added all 4 Tbsp). Whip until the frosting is smooth and silky.
Place one layer of cake on a cake stand or serving dish. Top with about 2/3 cup buttercream, spread evenly. Sprinkle some of the crumble on top. Repeat with remaining layers, but don't sprinkle crumble on the very top layer.
Frost and smooth the outside with a thin crumb coat (thicker on the very top). Chill for 20mins.
Using a small spoon, place dollops of ganache around the top edges of the cooled cake, allowing some to drip down. Fill in the top of the cake with more ganache and spread evenly with an offset spatula.***
Use a 1M tip to pipe dollops on top with remaining frosting and sprinkle some crumble in the middle.
* For the ganache, I let mine set overnight (with plastic wrap placed directly on the surface). It will be too thick at this point, so I microwave it for 5-10 secs and stir it until it's the right consistency. ** You will have some crumble leftover after using it in the cake. You can use this as a topping for ice cream or a crunch layer in another cake. *** You may have some ganache left over.