Preheat oven to 350F. Butter two 9" cake pans and line the bottoms with parchment paper. Set aside.

In a small bowl, combine the flour and potato starch. Mix well, then set aside.

Divide the egg whites into the bowl of a stand mixer fitted with the whisk attachment, and the yolks into a separate small bowl.

Add the cream of tartar to the whites to beat on Medium-High speed until frothy, about 2-3 minutes.

Gradually add the sugar in a steady stream while beating, then increase speed to HIGH for 1-2 minutes, until stiff peaks. The whites should be glossy and hold a firm peak.

In the yolks bowl, add the lemon zest, vanilla, and fine sea salt. Beat vigorously for 1 minute.

While the stand mixer is on Medium-Low speed, gradually pour in the yolks.

Shift a third of the flour mixture into the bowl, then use a spatula to fold it in. Be careful not to deflate the air whipped into the egg whites. Repeat sifting and folding in the remaining flour mixture until combined.

Heat up the heavy cream in either the microwave or a small saucepan JUST until warm to touch, about 10-30 seconds. Pour along the sides of the bowl, then fold together with a spatula.

Evenly divide the batter between the prepared cake pans.

Bake until evenly golden on top, about 20-22 minutes. Let cool in the cake pan for 20 minutes, then transfer to a wire rack to cool completely.

Place the egg whites and sugar together in a large metal bowl. Add a few inches of water to a small saucepan over medium high heat, bring to a boil, then place the metal bowl on top. This will act as your double boiler.

Whisk until the sugar has completely dissolved, about 5-6 minutes. Use your fingers to pinch a small amount together and feel for any graininess. Keep whisking until completely smooth.

Immediately transfer the bowl to your stand mixer fitted with the whisk attachment. Beat until stiff peaks and the bowl has cooled down to room temperature, about 10-15 minutes. If the bowl is still warm but the mixture has come to firm peaks, let cool off in the fridge for 10-15 minutes.

Switch the whisk to the paddle attachment. Add the softened butter a few tablespoons at a time to beat until smooth. The buttercream should come out thick, creamy, and silky smooth!

Fold in the vanilla and fine sea salt.

Add 3/4 cups of the passion fruit pulp to a high speed blender. Pulse until very smooth. Use a fine mesh strainer and spoon to strain the pulp into a saucepan. Add the sugar and lemon juice to the saucepan, as well.

Place the saucepan over medium heat, bring to a boil, stirring consistently, then simmer on LOW for 8-10 minutes, until slightly thickened. Let cool completely until cool to touch. You still want it slightly runny before adding to the cake, but you don't want it warm or it will melt the buttercream.

Add a dollop of the frosting to your cake plate then the first completely cooled cake layer. Use an offset spatula to add half of the meringue buttercream, then repeat with the second cake layer and remaining buttercream. Add the mostly cooled passion fruit curd on top. Use the back of a spoon to help evenly spread on top.