Sift the flour, baking powder and salt in a small bowl. Set aside.
In a large bowl, add the sugar and eggs and whisk until combined. Add the orange zest, vanilla extract and buttermilk and whisk until combined. Add the cooled melted butter and whisk to combine. Add the flour and carefully whisk until just combined, being careful not to over-whisk.
Prepare a 9x2 inch round baking pan (I use Magic Line), line the bottom with parchment paper or foil and butter the top of the lining and the sides of the pan.
Pour the batter into the pan, smooth the top with a rubber spatula, and bake for 30 minutes or until a toothpick inserted comes out clean.
Allow the cake to cool in the pan for 30 minutes before inverting it onto a wire rack and allowing it to cool completely.
To make the frosting: Combine the first 6 ingredients and beat with a mixer until combined and smooth. Add the powdered sugar and beat until combined and smooth.
Cut the cake in half. Spread some orange marmalade on the bottom half. Spread 1/4 of the cream cheese frosting on top of the marmalade. Invert the top half of the cake and place it on top of the bottom layer. Frost the cake with the remaining cream cheese frosting.