Heat the oven to 325°F. Spray 3 8-inch round pans with nonstick spray. Line the bottoms with parchment paper cut into a circle to fit the pan. Spray the parchment paper with nonstick spray and dust the pan lightly with cocoa powder. Tap out any excess and set aside.
In a medium bowl, combine the flour, baking soda, and salt. Stir with a whisk and set aside.
Add the chocolate to a small heatproof bowl and set it over a pot of barely simmering water. Stir continuously until melted and smooth. Remove from heat. Set aside to cool.
Using a stand mixer fitted with a paddle attachment or a handheld electric mixer, beat the butter and sugar together at medium-low speed until light and fluffy. Beat in the eggs, one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed. Beat in the vanilla. (The mixture may look curdled, this is normal and will come together once the rest of the ingredients are added.)
Add the cocoa powder and melted chocolate and mix until well incorporated. With the mixer set to low speed. gradually add half the flour mixture, followed by the coconut milk, and then the remaining flour mixture. Beat just until combined. The batter will be thick.
Divide the batter evenly between the 3 prepared pans. Use an offset spatula to spread the batter all the way to the edges of the pan and smooth out the surface. Bake for 28 to 30 minutes, or until a toothpick inserted into the center comes out with only a few dry crumbs. Cool the cake in the pans for 10 minutes, then invert the cakes onto a wire rack to cool completely. Peel the parchment paper off the bottoms before filling.
Make The Filling
Bring the coconut milk up to a simmer in a medium saucepan. In a separate bowl, whisk the egg yolks, sugar, cornstarch, and extract until blended and pale in color. Slowly pour in the hot milk while whisking continuously. Return the mixture back to the pot and whisk over medium-high heat until thickened, about 3 minutes. Pour the thickened pastry cream through a fine-mesh sieve then stir in the butter until fully melted. Place a piece of plastic wrap directly on top of the pastry cream, making sure it is touching the surface, and refrigerate until cooled completely. Stir in the shredded coconut.
Place one of the cake layers on a serving plate, bottom side up. Spread half the pastry cream over the top, leaving a ½-inch margin around the edge. Top with another cake layer. Repeat with remaining pastry cream and top with the third cake layer.
Make The Frosting
Prepare the coconut buttercream frosting according to the recipe instructions. Spread the frosting over the top and sides of the cake. Using a large cake scraper, remove frosting from the sides of the cake until the cake layers begin to show through.
Make The Glaze
Place the chocolate, butter, and corn syrup in a medium heatproof bowl set over a pot of barely simmering water. Stir until melted and smooth. Remove from heat and let cool for 5 minutes.
Spoon ¼ cup of chocolate glaze over the top of the cake. Drizzle the remaining glaze along the edge of the cake all the way around, allowing it to drip down the sides. Sprinkle shredded coconut over the top.
