Preheat oven to 375 degrees F. Grease two 9-inch cake pans with oil spray. Cut two 9-inch diameter circles of parchment paper and set one in the bottom of each pan. Set cake pans aside.
Place flour, salt and baking powder in a bowl and whisk to combine. Place the 3/4 cup butter, granulated sugar, vanilla and orange zest in a second bowl, or in the bowl of your stand mixer. Beat until well combined and lightened. Beat in the eggs, one at a time.
Add flour mixture in 3 parts alternately with orange juice, stirring the batter after each addition until smooth.
Divide and spoon the batter into the cake pans. Bake in the middle of the oven 25 to 30 minutes, or until cakes spring back when touched in the very centre. Cool cakes to room temperature, and then unmould.
To make frosting, place cocoa powder, 1/2 cup butter and water in a bowl, or in the bowl of your stand mixer. Beat until well combined and the cocoa is completely dissolved. Beat in the icing sugar.
To frost cake, place one of the cake rounds, domed side up, on a cake plate. Spread the top of that cake with the marmalade. Set on the second cake, flat-side up. Carefully, and as smoothly as you can, spread the frosting on the sides and top of the cake. Refrigerate the cake until ready to serve. Can be prepared several hours before needed. Garnish the cake, if desired, with half orange slices.