Preheat your oven to 350°F (175°C). Line your cupcake tins with cupcake liners.
In the bowl of your stand mixer, fitted with the paddle attachment, beat the butter and sugar together until light and fluffy, about 2 minutes.
Add in the egg whites and mix until fully combined. Add the vanilla and sour cream, mixing until incorporated, scraping down the sides of the bowl as needed. Don't worry if the mixture appears curdled.
In a medium-sized bowl, whisk together the cake flour, baking soda, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients.
Gently fold in the marmalade, being careful not to overmix the batter.
Divide the batter evenly among 14 lined cupcake tins.
Bake for about 18 minutes, or until the tops are golden yellow and spring back when lightly pressed. You can also check with a cake tester or toothpick—it should come out clean with just a few crumbs.
Allow the cupcakes to cool on a wire rack.