Preheat oven to 325ยบ. Butter 2 9-inch round cake pans, line with parchment or wax paper. Sift flour, baking soda, and salt. Beat butter, add sugar a little at a time until light and fluffy. Beat in eggs, orange zest, and vanilla. Beat in 1/2 of the dry ingredients with 1/2 of the buttermilk until combined. Add 1/2 of the remaining dry ingredients and remaining buttermilk and beat until well combined. Finally beat in remaining dry ingredients until smooth. Evenly divide into the cake pans. Bake 45 minutes until toothpick comes out clean. Cool cake 20 minutes. For orange syrup: stir orange juice and sugar until sugar is dissolved. Poke holes in cake at 1/2-inch intervals. Spoon syrup over cake. For Filling: Melt marmalade over moderate heat until melted. Cool 5 minutes. For Frosting: Beat cream to stiff peaks along with sugar. Add sour cream a little at a time. Whisk to spreading consistency. Arrange cake layer on plate. Spread 2/3 of marmalade over the top, smoothing. invert remaining layer and spread with remaining marmalade leaving 1 1/2 inch border around the edge. Frost border and sides leaving marmalade on top exposed. Chill 2 hours before serving.