The hardest step is this one—set your butter and eggs out on the counter and wait until they come up to room temperature. This will take at least an hour and is worth every minute.
Add the sugar or other sweetener and the softened butter to the bowl on your stand mixer or a nice big mixing bowl. Beat together with the paddle attachment on your stand mixer or your electric hand mixer or your favorite cooking spoon in a bowl for a long time. The idea is to produce a light and fluffy "cream" which can take at least five minutes, or as many as eight. If you've been bowling it, grab a coffee so your arm can recover.
To the creamed butter and sweetener, add the egg yolks one at a time and the eggs, one at a time and mix each one in thoroughly before going to the next. If you're nervous about introducing egg shells into the batter, break each egg into a small bowl and then add to the mix.
When all the eggs have been mixed in, add the vanilla and mix in.
In a separate bowl, mix together the flour, baking soda, and salt. Use your whisk and keep whisking for at least 30 seconds so the powder and salt are evenly distributed.
Now, add the flour mixture—one third of it at a time—to the wet mix and stir in carefully to avoid getting flour everywhere. If you use an electric mixer, run it SLOW. Add the second third of the dry mixture and continue mixing. Add the remaining third and more of the same.
Add the buttermilk—again one third at a time—and carefully mix until just combined, which means until the dry stuff disappears. Scrape down the bowl as needed—preferably with a rubber spatula so you get every bit—to get the stuff clinging up high down into the mix.
Finally, add the lemon zest and lemon and again, mix in thoroughly. For a different flavor, try other add-ins. For example, I recently made a "rhubarb-lemon" Bundt cake that was supposed to use fresh rhubarb. Couldn't find any, so used some canned strawberry-rhubarb pie filling. Weird, huh? It was great. Just be careful with the quantity of the add-in. Too much is too bad!
Now, it's time to bake! I made two 9" cake rounds, then, using frosting out of a can, made a lemon layer cake. Wow! It was amazing. I baked the rounds in a pre-heated 350° oven for 30 minutes, rotating the pans and re-positioning them in the oven after 15 minutes. I've learned my oven has hot spots.Next time, I'll pour the batter into a buttered 9"x13" baking dish for a sheet cake. Or, maybe I'll try a loaf pan and imitate a Starbucks lemon cake. All I can promise is that if you follow the instructions, you'll get a cake to be proud of and impress the mother-in-law, or whoever.