Sift flour, baking powder, baking soda, and salt into a large bowl and whisk well to blend evenly. It’s important to distribute the leavening agents uniformly to prevent large air pockets in the cake. Set aside
Combine butter, oil, sugar and vanilla extract in a large mixing bowl (or the bowl of a stand mixer) and mix together on medium-high speed using an electric handheld mixer until pale, light and fluffy. This will take 3-5 minutes. Scrape down the bowl halfway through mixing to ensure there are no bits of butter that haven’t been blended.
Add the egg whites 2 at a time and mix for 1-2 minutes until well combined between each addition. Scrape down the bowl again. Add sour cream and mix it in evenly.
Reduce the mixer speed to low and then add half of the flour mixture until just combined then add half of the milk. Scrape down the bowl and repeat adding half of the remaining flour, followed by the rest of the milk and finally gently mix in the rest of the flour. Be sure to stop and scrape down the bowl frequently so the batter is even. It’s important not to over-mix, however undermixing will mean the batter is not homogeneous and it will not bake evenly.
Divide the batter evenly between two pans (about 600g per pan) and spread it out evenly using a small offset spatula, tap the pans on the countertop to force out any large air bubbles and bake for 25-30 minutes or until evenly golden and a toothpick comes out clean.
Prepare the frosting. Follow this recipe for Vanilla Whipped Ganache and spread about 1 cup over one layer of white vanilla cake. Place the other layer on top and cover the cake with the remaining frosting. If serving later, keep the cake covered under a cake dome, or keep refrigerated if serving the next day.