In a standing mixer fitted with a paddle attachment or using a hand mixer, beat together, 4 egg yolks and 1⅓ cup sugar. Beat until fluffy and visibly lighter in color, approximately 3 to 5 minutes.
Add to mixing bowl, 1 cup pumpkin puree, 2 tablespoons vanilla, 4 tablespoons bourbon or whiskey, and 2 - 3 tablespoons of vegetable oil. NOTE: The vegetable oil helps blend the pumpkin puree and gives it a smoother texture, add gradually and use as little as possible. Mix until combined, scrapping down the sides and bottom of the bowl.
In the same bowl add remaining dry ingredients in parts, 4 tablespoons lemon zest, 2¼ cups flour, 4 teaspoons baking powder, 2 teaspoons baking soda, and 2 tablespoons salt. Mix well. Slowly add the browned butter mixture and set aside.
In a clean mixing bowl, beat 4 egg whites until medium-stiff peaks are formed.
Fold egg whites in gently 2 or 3 batches at a time into the batter. Note: Taking care to keep as much air in the batter as possible.
Fold in the apples until combined and immediately pour batter into your prepared pan(s).
Bake for 55 to 75 minutes or until a cake tester inserted into the bottom center of the cake comes out clean.