Use a pastry brush to grease a 12-cup Bundt or cast-iron fluted cake pan with a thin layer of vegetable shortening. Dust with flour and tap out the excess.
Use a sifter or metal strainer to sift the flour into a medium size bowl. Add baking soda and salt, set aside.
Separate the eggs, carefully. Do not get any egg yolk into the egg whites or they will not whip. Whip the egg whites with the cream of tartar in a clean, grease free bowl. Whip until you have a fairly stiff peak. Set aside.
In a large mixing bowl, using either a stand mixer or an electric hand mixer, cream the butter until light and fluffy, about 2-3 minutes.
Add the sugar gradually, beating thoroughly. Scrape the sides and bottom of the bowl with a rubber spatula occasionally.
Add the egg yolks, one at a time. Scrape the bowl again. Add the lemon juice, lemon zest, buttermilk and lemon oil. Combine.
Gradually add the dry mixture to the cake batter and incorporate only until the batter is smooth and combined.
Use a rubber spatula to gently fold the whipped egg whites into the cake batter.
Use a rubber spatula to scoop the cake batter into the prepared cake pan. Gently spread the batter around evenly.
Place the cake in a preheated 325 degree oven for about 1 hour 10 minutes. Or until a toothpick when inserted comes out mostly clean, with some moist crumbs.
Remove the cake from the oven and let cool for 10 minutes in the pan. Remove the cake from the pan and let it cool completely on a baking rack. Once cooled, then drizzle with lemon icing.