For the cake
1 cup unsalted butter, softened, more for greasing the pans
3 1/4 cups flour, more for dusting the pans * I used all purpose flour. The original recipe calls for cake flour
1 tablespoon baking powder
1 teaspoon salt
2 2/3 cups granulated sugar
5 large eggs, at room temperature
4 large egg yolks, at room temperature
2/3 cup vegetable oil
1 tablespoon grated orange zest
2 teaspoons vanilla extract
1 cup buttermilk, at room temperature
For the orange syrup
1 cup orange juice * the original recipe calls for freshly squeezed0
1/4 cup granulated sugar
For the filling
1 (12-ounce) jar orange marmalade
For the frosting
1 cup heavy cream, chilled
4 tablespoons granulated sugar
1 cup sour cream, chilled
The cake. Preheat the oven to 350°F. Lightly butter three 9-inch round cake pans, line them with parchment paper, then lighly butter and flour the paper, shaking out the excess.
Sift the flour, baking powder, and salt into a large bowl. Sift a second time into another bowl. In the bowl of an electric mixer, beat the butter on medium speed until light in color, about 4 minutes. Add the 2 2/3 cups sugar in a steady stream with the mixer running. Beat until light and fluffy, about 4 minutes. Add the eggs and yolks, one at a time, beating well after each addition. Be sure to stop at least once to scrape down the batter from the sides of the bowl. After all the eggs have been added, continue to beat on medium speed for 2 more minutes. With the mixer on low speed, add the oil and beat for 1 minute. In a small bowl, combine the orange zest, vanilla, and buttermilk. Using a rubber spatula, fold in half of the dry ingredients. Scrape down the sides of the bowl and add half of the buttermilk mixture. Fold in the remaining dry ingredients, scrape down the sides, and add the remaining buttermilk.
Pour the batter among the prepared pans, smooth the surface, rap each pan on the counter to expel any air pockets or bubbles, then place in the oven. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans on racks for 20 minutes.
The orange syrup. In a small bowl, stir together the orange juice and 1/4 cup sugar until the sugar is dissolved. While the cakes are still in the cake pans, use a toothpick or skewer to poke holes at 1/2-inch intervals in the cake layers. Spoon the syrup over each layer, allowing the syrup to be completely absorbed before adding the remainder. Let the layers cool completely in the pans.
The filling. Heat the marmalade in a small saucepan over medium heat until just melted. Let cool for 5 minutes.
The frosting. In a chilled mixing bowl, using the wire whisk attachment, whip the heavy cream with the 4 tablespoons sugar until stiff peaks form. Add the sour cream, a little at a time, and whisk until the mixture is a spreadable consistency.
To assemble the cake. Invert one of the cake layers on a cake plate and carefully peel off the parchment. Spread one-third of the marmalade over the top, smoothing it into an even layer. Invert the second layer on top of the first, peel off the parchment, and spoon another third of the marmalade on top. Place the third cake layer on top, remove the parchment, and spoon the remaining marmalade onto the center of it, leaving a 1 1/4-inch border around the edges. Frost the sides and the top border with the frosting, leaving the marmalade on top of the cake exposed.
Chill for at least 2 hours before serving, overnight if possible.