Preheat your oven to 350 degrees Fahrenheit. Line the bottoms of three 8-inch round cake pans with parchment paper, then spray the pans with nonstick cooking spray. Set aside.

In a small saucepan over medium-low heat, combine the water and butter. Once the butter is melted and the mixture is smooth, remove from heat. Whisk in the cocoa powder and chopped dark chocolate until fully melted and smooth. Stir in the brown sugar, vanilla, and salt.

In a large bowl, whisk together the eggs and egg yolks. Slowly drizzle the egg mixture into the chocolate mixture, whisking constantly, until fully incorporated.

Sift the flour and baking soda together, then gradually fold them into the batter until just combined. Be careful not to overmix; a few streaks of flour are fine.

Divide the batter evenly among the prepared pans. Bake for 25 to 30 minutes, until the cakes spring back when gently pressed and a toothpick inserted in the center comes out clean or with a few moist crumbs. Cool the cakes in their pans for 1 hour, then run a knife around the edges to loosen and turn them out onto a wire rack. Let cool completely before frosting.

Reduce the oven temperature to 325 degrees Fahrenheit. Spread the shredded coconut in a thin layer on a baking sheet and toast for 5 to 10 minutes, until lightly golden. Let cool completely.

In a large bowl or stand mixer fitted with the paddle attachment, beat the room-temperature butter on medium speed until light and fluffy, about 4 minutes. Add about one-third of the sweetened condensed milk and beat until smooth and homogeneous.

Add the vanilla, salt, and another third of the sweetened condensed milk. Beat until combined. Add the remaining sweetened condensed milk and 1/2 cup powdered sugar, and beat until smooth. If the frosting is too soft, chill it in the refrigerator for 15 to 20 minutes, or add up to 1/2 cup more powdered sugar.

If your cakes have domed tops, use a serrated knife to level them so they are flat. Place one cake layer on a serving plate. Spoon about one-third of the frosting into a piping bag fitted with a round tip, and pipe it evenly over the cake layer. Sprinkle generously with toasted coconut. Repeat with the remaining layers, finishing with a layer of frosting and coconut on top. Garnish with fresh blackberries if desired.