Preheat oven to 350F (175C) and prepare a 12 count cupcake tin by lining with paper liners. Set aside.
Place your egg whites in a completely clean, dry, and grease-free bowl. Using an electric mixer, beat on high-speed until stiff peaks form.¹ Set aside.
In a separate bowl, use an electric mixer to beat together butter, oil, and sugar. Beat until ingredients are creamy and well-combined.
Scrape down the sides and bottom of the bowl and then stir in your vanilla extract and sour cream, stirring until combined.
In a separate bowl, whisk together your flour, baking powder, baking soda and salt.
Using a spatula and stirring by hand (don’t use your mixer or you run the risk of over-beating the batter), stir the dry ingredients into the wet (butter/sugar mixture) until completely combined. Scrape the sides and bottom of the bowl to ensure ingredients are well-combined. Set aside.
Using a spatula, gently fold your egg whites into your batter. While you do not want to over-mix this batter, it is important that the egg whites are completely combined. Use a gentle hand and stir until you can’t see any streaks of egg whites in the batter.
Divide batter evenly into prepared cupcake tin.
Bake on 350F (175C) for 17-19 minutes, or until a toothpick inserted in the center comes out mostly clean or with a few moist crumbs (no wet batter).
Allow cupcakes to cool completely before carving and frosting: