Step 1

Adjust oven rack to middle position and heat oven to 350 degrees. Grease 9-inch round cake pan, line with parchment paper, then grease parchment. Grate 2 teaspoons zest from 1 orange; set aside. Using your hands, peel oranges. Using sharp chef's knife or serrated knife, trim ends and slice oranges crosswise ¼ inch thick, removing any seeds.

Step 2

Pour 4 tablespoons melted butter over bottom of prepared pan and swirl to evenly coat. Whisk ½ cup sugar, cornstarch, and ⅛ teaspoon salt together in bowl, then sprinkle evenly over melted butter in pan. Arrange orange slices in single layer over sugar mixture, nestling slices snugly together and pressing them flat (you may have fruit left over).

Step 3

Whisk flour, baking powder, and remaining ½ teaspoon salt together in large bowl. Whisk sour cream, eggs, vanilla, orange zest, and remaining 1 cup sugar in second large bowl until smooth, about 1 minute. Whisk remaining 6 tablespoons melted butter into sour cream mixture until combined. Add flour mixture and whisk until just combined.

Step 4

Pour batter over oranges in pan and smooth top with rubber spatula. Bake until deep golden brown and toothpick inserted in center comes out clean, 50 to 55 minutes.

Step 5

Let cake cool in pan on wire rack for 20 minutes. Run knife around edge of pan to loosen cake, then invert cake onto serving platter. Discard parchment. Let cake cool for at least 1 hour.

Step 6

Microwave marmalade in bowl until fluid, about 20 seconds. Using pastry brush, brush marmalade over top of cake. Serve.