Preheat the oven to 350 degrees F. Grease and flour three 8-inch cake pans and line the bottoms with parchment paper.

Sift together the cake flour, baking powder, and salt into a large bowl.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed for 2 minutes. Add the sugar and mix on medium-high until light and fluffy, 3 to 5 minutes. Stop the mixer and scrape down the bowl.

Turn the mixer to medium-low and add the vanilla. Add the egg yolks and eggs one at a time, mixing until each is incorporated before adding the next. Mix until combined. Stop the mixer and scrape down the bowl.

Turn the mixer to low and add the flour mixture in three batches, alternating with the milk, beginning and ending with the flour mixture. After the last streaks of the flour mixture are combined, mix on medium for no more than 30 seconds. Stop the mixer and fold in the chocolate by hand.

Evenly divide the batter among the three prepared pans. Bake for 28 to 35 minutes, until a toothpick inserted into the center of each cake comes out clean. Let the cakes cool on a wire rack for 10 to 15 minutes before removing from the pans. Allow the cakes to cool completely, right-side up, on the wire rack before removing the parchment.

Put the egg whites and sugar in the bowl of a stand mixer. Gently whisk them by hand to combine. In a medium saucepan, bring an inch or two of water to a simmer over medium-low heat. Place the mixer bowl on top of the saucepan to create a double boiler. Heat the egg mixture, whisking intermittently, until it reaches 160 F on a candy thermometer.

Carefully affix the mixer bowl to the stand mixer. Beat the egg white mixture on high speed for 8 to 10 minutes, until the mixture holds medium-stiff peaks and the outside of the bowl has cooled to room temperature.

Remove the whisk attachment and add the paddle attachment. Turn the mixer to medium-high and add the butter and beat until the buttercream is silky smooth, 3 to 5 minutes.

Add 6 tablespoons of the passion fruit concentrate a few tablespoons at a time and mix until combined, then taste and add the remaining passion fruit concentrate to your liking.

Place one cake layer on a cake board or serving plate. Spread about ¾ cup of the passion fruit buttercream with an offset spatula. Top with a second cake layer and repeat; place the final cake layer on top. Crumb coat the cake with the buttercream and chill in the fridge for 15 minutes.

Smoothly frost the cake with a thin layer of buttercream.

To decorate the cake with the ombre shell piping, divide the remaining buttercream into three bowls. Tint each of the batches of buttercream to different degrees of yellow, going from darker to lighter. Fill a piping bag fitted with a small star tip with the tinted buttercream. Pipe a continuous row of shells around the bottom of the cake. After about three rows, empty the piping bag and fill it with the slightly darker shade. Repeat until you've worked your way all of the way up the cake.

If you like, you can add some fresh passionfruit puree to the very top.