For the orange filling: In a medium saucepan, whisk together sugar, cornstarch and salt. Stir in orange zest, orange juice, and lemon juice. Bring mixture to a boil over medium heat, stirring frequently, and allow to boil about 1 minute (mixture should thicken slightly and become translucent).
Meanwhile, in a bowl, whisk together egg yolks and egg until very well blended. While mixing egg mixture, slowly pour 1/4 cup of the hot orange juice mixture over eggs and stir until well blended.
Remove saucepan from heat (once it's boiled as listed above) and while stirring mixture in saucepan, slowly pour egg mixture into saucepan. Reduce burner temperature to medium-low heat and return saucepan to burner and cook, stirring constantly (and whisking rather vigorously to prevent curdling) until mixture thickens well, about 4 minutes.
Remove from heat, add butter and stir until combined. Pour into a bowl, cover with plastic wrap pressing directly against surface of filling, and chill 2 hours.
For the cake: Preheat oven to 350 degrees F. Butter and flour three 9-inch round cake pans (lightly butter sides), set aside.
To the bowl of an electric stand mixer fitted with the paddle attachment (or using a bowl and electric hand mixer), add 1 1/3 cups granulated sugar, baking powder, salt and sift cake flour into bowl. Pour in canola oil, egg yolks (while reserving the whites in a separate mixing bowl) and orange juice.
Blend mixture on medium-high speed until smooth, about 3 - 4 minutes (slightly less if using a paddle attachment that has the scraper), occasionally scraping down sides and bottom of bowl. Blend in orange zest and vanilla extract.
In a large mixing bowl, using an electric hand mixer (or if you have two stand mixer bowls you can use your stand mixer with the whisk attachment), whip 5 reserved egg whites with 1/2 tsp cream of tartar on moderately high speed until stiff (but not dry) peaks form.
Add half of the whipped egg whites to the batter and fold until partially combined, then add remaining half of the whipped egg whites and fold just until combined (be careful not to overfold and deflate the eggs).
Divide batter evenly among prepared baking pans and bake in preheated oven 17 - 19 minutes until toothpick inserted into center of cake comes out clean. Allow to cool in cake pans 10 minutes, then invert onto wire racks to cool completely before frosting with meringue.
For the meringue: Add 6 egg whites (don't allow one drop of egg yolk) and sugar to a very clean heatproof bowl and set over a saucepan of barely simmering water. Heat mixture, stirring constantly, until it registers 160 degrees on a candy thermometer, about 3 - 5 minutes.
Remove bowl from saucepan, add 1/4 tsp cream of tartar and beat mixture with an electric stand mixer on medium-high speed until stiff glossy peaks form, about 7 - 9 minutes (possibly longer in a humid environment).
To assemble cake: Trim tops of each cake off to form an even layer. Spread half of the chilled orange filling over 1 layer of the cake (crumb/top side up, drop in a large dollop and spread outward so it doesn't get all crumby), leaving about a 1/2-inch rim of cake uncoated (because once the other layers are added the weight will slightly push the filling outward).
Top with another layer of the cake (crumb/top side up), followed by remaining half of the orange filling, leaving a 1/2-inch rim of cake uncoated. Top with last layer (crumb/top side down) and spread meringue over entire cake.
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