Preheat the oven to 350°F with a rack in the center. Line three 6-inch round cake pans with parchment paper, spray with cooking spray (underneath paper as well). Set aside.
Measure flour by spooning into a measuring cup and lightly sweeping the excess off. Or you can weigh the flour.
In a small bowl, combine the flour, salt and baking powder. Whisk together until combined. Set aside.
In a large bowl, beat the eggs, sugar and lemon oil with an electric mixer using the whisk attachment until thickened and a light golden color. About 2 minutes at medium high speed. The batter should fall in thick ribbons from the beaters.
Add the dry ingredients to the batter and mix by hand, just enough until combined. Scrape bottom and sides of the bowl, then mix again briefly to fully incorporate.
With mixer on low, slowly add boiling water, melted butter and oil. Let it mix until batter is combined.
Divide the batter evenly between the 3 pans.
Bake the cakes until a toothpick inserted into the center comes out clean and the top feels set. About 40-45 minutes.