2Preheat oven to 350 degrees. Lightly butter three 9-inch round cake pans, line them with parchment paper, then lightly butter and flour the paper, shaking out any excess.
3Sift 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.
4Add 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 of the eggs have been added, continue to beat on medium speed for 2 more minutes.
5With the mixture on low speed, add the oil and beat for 1 minute.
6In a small bowl, combine the orange zest, vanilla, and buttermilk.
7Using 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.
8Pour the batter among the three prepared pans, smooth the surface, rap each pan on a counter to expel any air pockets or bubbles, then place in oven.
9Bake for 30-35 minutes, or until a toothpick, inserted in the middle, comes out clean. Let the cakes cool in the pans on racks for 20 minutes.
10THE ORANGE SYRUP:
11In 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.
12Spoon the syrup over each layer, allowing the syrup to be completely absorbed before adding the remainder. Let the layers cool completely in pans.
14Heat the marmalade in a small saucepan over medium heat until just melted. Let cool for 5 minutes.
16In a chilled mixing bowl using a wire whisk attachment, whip the heavy cream with the 4 Tbs. of sugar until stiff peaks form. Add the sour cream, a little at a time, and whisk until the mixture is a spreadable consistency.
17TO ASSEMBLE THE CAKE:
18Invert 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 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 edge.
19Frost the sides and top border with the frosting, leaving the marmalade on top of the cake exposed. Or, if you prefer, frost the entire cake first, adding the marmalade as a garnish on top.