Preheat oven to 325 degrees F. Butter two 9-inch round cake pans, line with parchment or waxed paper, and butter and flour the paper, shaking out the excess.
In a bowl, sift the flour, baking soda, and salt. In a bowl with an electric mixer, beat the butter until combined, add the sugar, a little at a time, and beat the mixture until light and fluffy. Beat in the eggs, orange zest, and vanilla. Beat in 1/3 of the dry ingredients alternately with 1/2 of the buttermilk until combined well. Add half the remaining dry ingredients and the remaining buttermilk and beat until combined well. Finally, beat in the remaining dry ingredients until mixture is smooth.
Evenly divide the batter between the pans, smooth the surface, rap each pan on the counter to expel any air pockets or bubbles, then transfer to the oven. Bake for 45 minutes or until a cake tester inserted in the center comes out clean. Transfer to racks and cool in the pans for 20 minutes.
To Make the Orange Syrup: In a bowl, stir together the orange juice and sugar until sugar is dissolved. With a toothpick or wooden skewer, poke holes at 1/2-inch intervals in the cake layers and spoon the syrup over each layer, allowing the syrup to be completely absorbed before adding the remaining. Let layers cool completely.
To Make the Filling: In a small saucepan set over moderate heat, heat the marmalade until just melted. Let cool 5 minutes.
To Make the Frosting: In a bowl, whisk the heavy cream with the sugar until it forms firm peaks. Add the sour cream, a little at a time, and whisk until of spreading consistency.
To Assemble the Cake: Arrange one of the layers on a cake plate, carefully peel off the waxed paper, then spread 2/3 of the marmalade over the top, smoothing it into an even layer. Invert the remaining layer onto the top of the first layer, peel off the waxed paper and spoon the remaining marmalade onto the center of it, leaving a 1 1/4-inch border around the edge.
Frost the sides and top of the border with the frosting, leaving the marmalade on top of the cake exposed. Or if you prefer, frost the entire cake, adding the marmalade as a garnish on top. Chill for at least 2 hours before serving.