How to Make It
Preheat the oven to 350°. Spray two 8-inch round cake pans with vegetable-oil spray and line the bottoms with parchment paper. Spray the paper.
In a large bowl, mix the flour with the sugar, baking powder, salt and orange zest. Using an electric mixer at medium-low speed, beat in the eggs, orange juice, butter and water until blended, about 2 minutes. Divide the batter between the prepared pans and bake for about 35 minutes, or until a toothpick inserted in the center of each cake comes out clean. Let cool in the pans for 10 minutes, then invert the cakes onto a rack to cool completely. Peel off the parchment paper.
In a medium saucepan, combine orange juice, sugar, flour, orange zest and salt and cook over moderate heat, whisking constantly, until very thick and just beginning to boil, about 8 minutes. Off the heat, whisk in the butter and lemon juice, then transfer to a bowl. Place a sheet of plastic wrap directly on the surface of the filling and refrigerate for 30 minutes, or until chilled.
In a small saucepan, combine the sugar and water and cook over moderate heat until the syrup reaches 240° on a candy thermometer; this can take up to 12 minutes. Meanwhile, in the bowl of a standing electric mixer, beat the egg whites with the cream of tartar until soft peaks form. With the machine on medium-low speed, slowly pour in the hot sugar syrup against the side of the bowl. Once all of the sugar syrup has been added, increase the speed to medium-high and beat until the meringue icing is fluffy, glossy and at room temperature, about 8 minutes.
Using a serrated knife, split each cake in half horizontally. Place a cake layer cut side up on a plate. Spread one-third of the orange filling on the layer and cover with the top half of the cake. Spread another third of the orange filling on top. Repeat with the remaining cake layers and orange filling, ending with a cake layer. Spread the meringue icing all over the top and side of the cake. Let stand for 1 hour before serving.
The cake can be refrigerated, uncovered, overnight.