This is a classic Victorian era cake made with sponge layers, sliced oranges for a filling and then an orange buttercream frosting. Leftovers will hold in the refrigerator fairly well for up to two days although the cake is best eaten on the day it is made.
For the Cake:
½ cup plus 2 tablespoons cake flour
¼ cup all purpose flour
1/2 teaspoon baking powder
¼ teaspoon salt
3 tablespoons milk
2 tablespoons unsalted butter
½ teaspoon vanilla extract
5 large eggs, at room temperature
¾ cup granulated sugar
1 teaspoon lemon zest
2 tablespoons lemon juice
For the Buttercream:
12 tablespoons unsalted butter, softened
¾ teaspoon grated orange zest (you can use one of the oranges needed to finish the cake)
1 ½ cups confectioners’ sugar (5 ounces)
2 teaspoons lemon juice
½ teaspoon vanilla extract
1 tablespoon heavy cream
To finish the cake:
2 ripe sweet oranges
1. For the cake: Heat the oven to 350 degrees and adjust a rack to the center position. Lightly butter two 9 inch round cake pans and line the bottoms with parchment paper. Whisk flours, baking powder and salt in a medium bowl (or sift onto waxed paper). Heat milk and butter in a small saucepan over low heat until butter melts. Off heat add vanilla, set aside.
2. Separate 3 of the eggs, placing whites in bowl of standing mixer fitted with a whisk attachment. Reserve the 3 yolks plus remaining 2 whole eggs. Beat the 3 whites on high speed until foamy. Gradually add 6 tablespoons of sugar while beating to soft, moist peaks. Do not overbeat since stiff, dry whites will be difficult to incorporate into the batter. Transfer whites to a large bowl and add the yolks and remaining whole eggs to the mixing bowl. Beat yolks and whole eggs with the remaining 6 tablespoons sugar at medium-high speed until eggs are very thick and a pale lemon color, about 5 minutes. Add the lemon zest and juice and beat to combine, about 30 seconds. Return beaten whites to the bowl; sprinkle the flour mixture over eggs and whites. Mix on lowest speed for 10 seconds. Remove bowl from mixer, make a well in one side and pour melted butter mixture into well. Fold mixture with a large rubber spatula until homogeneous.
3. Immediately pour batter into prepared pans. Bake until cake tops are lightly browned, feel set, and spring back when touched, about 16 minutes. Place one pan on a kitchen towel, run a thin knife around perimeter and cover pan with a large plate. Invert pan and plate, then remove pan from cake. Peel away parchment paper and invert cake onto cooling rack. Repeat with second cake. Cool completely.
4. For the buttercream: In the bowl of electric mixer fitted with the whisk attachment, beat butter on medium high speed until smooth, about 20 seconds. Add zest and beat to combine, about 15 seconds. Add confectioners sugar and salt and beat at medium low until the sugar is moistened, about 45 seconds. Scrape down sides of bowl and add lemon juice, vanilla and heavy cream and beat at medium speed until incorporated, about 15 seconds. Increase speed to medium high and beat until light and fluffy, about 4 minutes, scraping sides of bowl once or twice.
5. To finish the cake: Using a sharp knife cut away orange peel carefully until no traces of pith remain. Cut oranges horizontally into ¼ inch slices and then cut slices into 1 inch strips.
6. Place one layer of cake on a cake round or serving plate. Arrange orange pieces in a single layer, slices slightly overlapping. Spread a very thin layer of buttercream on the bottom side of the second layer (you should be able to see the cake through the buttercream) and place on top of the orange slices. The top of the second cake layer should now be on top. Spread the remaining buttercream evenly over the top and sides of the cake. Serve immediately or refrigerate until 20 minutes before serving.