This cake was a sincere mistake! I had planned to pair a chocolate ganache with a coconut macaroon biscuit. As I was making it, I realized I had added too much egg white and was running out of coconut. But I decided to spread it out on a sheet pan and bake it anyway. The result? A delicious and very sweet mistake indeed.

SERVES 6 TO 8

COCONUT SPONGE

Nonstick cooking spray

1½ cups sugar

4 large eggs

3⅔ cups unsweetened dried shredded coconut

GANACHE

10½ ounces bittersweet chocolate, chopped

3½ ounces milk chocolate, chopped

1⅔ cups heavy cream

GARNISH

1 cup unsweetened dried shredded coconut, toasted

FOR THE COCONUT SPONGE

Heat the oven to 350°F. Spray the bottom and sides of a 17½ × 12½-inch rim med baking sheet with cooking spray and line it with parchment paper.

Fill a medium saucepan one-third full of water and bring it to a simmer. In the bowl of an electric mixer fitted with the whisk attachment, whisk the sugar and eggs until pale yellow. Place the bowl over the pan of simmering water and cook, whisking constantly, until the egg mixture is warm to the touch. Return the bowl to the mixer and beat the egg mixture on high speed for about 5 minutes, or until it has doubled in volume. Using a rubber spatula, fold in the coconut just until blended.

Pour the batter onto the prepared baking sheet, and using the rubber spatula, spread it evenly in the pan. Bake for 20 to 25 minutes, or until the top of the cake is light golden brown and a toothpick inserted in the center comes out clean.

Remove the baking sheet from the oven, place it on a wire rack, and let cool for 15 minutes.

Run a small sharp knife around the sides of the pan to loosen the cake. Place a wire rack over the cake and invert. Carefully peel off the parchment paper (the cake is extremely delicate). Cool the cake completely.

FOR THE GANACHE

Combine the bittersweet chocolate and the milk chocolate in a large heatproof bowl. In a medium saucepan, bring the cream to a boil. Immediately pour the hot cream over the chocolate. Whisk until the chocolate is completely melted and smooth. Cover the ganache with plastic wrap, pressing it directly against the surface, and refrigerate for about 4 hours, or until it is firm enough to spread.

TO ASSEMBLE THE CAKE

Trim off any uneven edges from the cake and cut it crosswise into three equal rectangles; each should measure about 5 by 10 inches. Place one of the rectangles on a serving platter. Using a small metal offset spatula, spread a generous layer of ganache over the top of the cake layer. Repeat with the remaining cake layers and ganache. Spread the remaining ganache over the sides of the cake. Sprinkle the toasted coconut over the top and sides. If not serving immediately, refrigerate the cake. (The cake can be made up to 1 day ahead. Bring to room temperature before serving.)