Preheat oven to 350F (175C) and lightly grease and flour a 9x5” (23x12cm) bread pan or line with a parchment paper sling. Set aside.

In a large mixing bowl, combine sugar, butter, and lemon zest and use an electric mixer to beat until light, fluffy, and well-creamed (about 3-4 minutes on medium-high speed).

Stir in the oil until well combined.

In a separate bowl or large measuring cup, whisk together sour cream, egg yolks, lemon juice, and vanilla extract until well-combined, and then stir into the butter mixture on low-speed until fully incorporated. Set aside.

In a separate mixing bowl, whisk together cake flour, baking powder, and salt. Gently fold the dry ingredients into the wet until completely combined.

With mixer on low-speed, alternate adding the flour mixture and sour cream mixture to the batter, starting and ending with the flour mixture.

In a separate, medium-sized clean, dry, grease-free mixing bowl, use an electric mixer to beat the egg whites on low-speed, then gradually increase to high and beat until you have stiff peaks.

Add the egg whites to the batter and use a spatula to gently fold together until batter is just combined, smooth and uniform. Pour evenly into prepared loaf pan.

Transfer pan to center rack of 350F (175C) preheated oven and bake for 74 minutes or until a wooden skewer inserted in the center comes out clean or with a few moist crumbs.

Allow the loaf to cool in the baking pan for 15 minutes before running a knife around the edges to loosen and carefully removing to a cooling rack.

Whisk together powdered sugar, 1 Tablespoon lemon juice, butter, and vanilla extract. Continue to add lemon juice as needed until mixture is smooth. Spread or pour evenly over loaf.

Allow loaf to cool and glaze to set for at least 30 minutes after frosting before slicing and serving.