Preheat oven to 325.
Butter two 9 inch round cake pans, line with parchment paper or wax paper and butter and flour paper, shaking out the excess.
In a bowl, sift the flour, baking soda and salt.
In another bowl, beat the butter with an electric mixer until smooth.
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 ½ of the wet until combined well.
Add half of 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 into the center comes out clean.
Transfer to racks and cool in the pans for 20 minutes.
To make the orange syrup:
Meanwhile, make the orange syrup: in a bowl, stir together the orange juice and sugar until sugar is dissolved.
When cake has cooled slightly but is still warm, with a toothpick or wooden skewer, poke holes at ½ inch intervals in the cake layers.
Spoon the syrup over each layer, allowing the syrup to be completely absorbed before adding the remainder.
Let layers cool completely.
To make the filling.
In a small saucepan set over moderate heat, beat the marmalade until just melted.
Let cool for 5 minutes.
To assemble the cake.
Arrange one of the layers on a cake plate and carefully peel off the wax paper.
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, peel off the wax paper and spoon the remaining marmalade onto the center of it, leaving a 1- ¼ inch border around the edge.
Frost the sides and the top 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 in the center of the top of frosting.
Chill for at least 2 hours before serving.