Preheat oven to 350°F. Grease 3, 8-inch cake pans and line them with parchment rounds.
Add the unsalted butter and granulated sugar into the bowl of a stand mixer. Beat on a high speed with a whisk attachment for a few minutes, scraping down the sides of the bowl as need with a rubber spatula. The mixture will become lighter in color as air is whipped into it.
Add the egg whites and mix at a medium speed until they're incorporated.
Sift the dry ingredients (cake flour, baking powder, ground cinnamon and salt) into a separate bowl. Add half of the dry ingredients into the butter mixture and mix on a low speed until just incorporated.
Add the sour cream, vanilla extract, and oil, and mix at a low speed until combined, scraping the sides and bottom of the bowl as needed.
Mix in the remaining dry ingredients on a low speed.
Pour the batter evenly between the prepared cake pans. I like to use a digital kitchen scale to weigh my pans, to make sure my layers will be the same height.
Bake for 31-34 minutes, or until a toothpick comes out with a few moist crumbs. Rotate pans halfway through to help them bake evenly.
Allow the pans to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan and flip the layers onto a wire rack to finish cooling.
Once the cake layers have fully cooled, use a serrated knife to level the tops. I chose to cut each cake layer in half horizontally, to make six thin cake layers but this is optional.