Spray with baking spray or grease with butter the bottom of 3 8inch round pans, place parchment paper on the bottom and spray with baking spray again.
In the bowl of an electric mixer, whisk together melted butter and sugar until smooth. Add vanilla extract and each egg one at a time on low speed and whisk until well combined.
Using a large rubber spatula, or with the mixer on low speed, gently stir in flour, cocoa, baking powder and salt.
Spread batter equally into the pans and bake for 25-30 minutes until set.
Remove from oven and let cool completely into the pans before assembling the cake.
Run a knife thru the sides of the pans to release the brownie layers. Carefully, cover one pan with a cake lifter, and flip the brownie layer onto the cake lifter. Place on the counter and repeat with the remaining brownie payers.
For the Coconut Filling:
Place butter, walnuts and coconut in a large bowl and set aside.
In a medium sauce pan, on low/medium heat, stir together the heavy cream, sugar and egg yolks until the mixture begins to thicken and coats the back of a spoon (180 degrees F.). Pour the hot custard immediately onto the walnut-coconut mixture and stir until the butter is melted. Cool completely to room temperature before topping the brownie layers.
For the Vanilla Mascarpone Buttercream:
Place softened butter and mascarpone into the bowl of an electric stand mixer that has been fitted with the wire attachment. Turn the mixer on a medium setting and cream until smooth and combined, 2 - 3 minutes.
Add sugar, ½ a cup at a time. Add vanilla bean seeds and a pinch of salt and whisk until well-incorporated.
If the frosting is too thick add heavy cream one tablespoon at a time until it has reached the desired consistency.
For the Chocolate Ganache:
Place the chocolate and corn syrup in a medium bowl. Heat the cream in a small saucepan over medium heat until it just begins to boil. Remove from heat and pour over the chocolate. Let stand one minute, then stir until smooth. Cool to room temperature.
Assemble the Cake:
Using a cake lifter, place first cake layer on a cake plate.
Top with a half of the coconut walnut filling, spread it evenly. Top the coconut mixture with 1/3 of the frosting, also spread evenly. Repeat the process with the second brownie cake layer. Third (top) layer, is covered in frosting only, no coconut mixture.
Pour the chocolate ganache on top of the cake, distribute evenly and also ice the sides of the cake while the ganache is dripping down.
Decorate with frosting if you have any remaining and chocolate sprinkles.
Chill the cake for at least 2 hours before serving.