Remember that the two key steps in preparing the cake are 1) whipping the eggs very well, and 2) incorporating the flour slowly (sifting on top of the batter).
This recipe makes one 9 inch diameter cake. Using larger pans is fine, but will result in thinner cakes. If using a larger pan, you can simply increase the quantities. I usually prepare several pan di Spagna at the same time. Doing so saves time in cleaning stuff, preparing the ingredients, and so on. However, one must clean the mixing bowl and the mixing blades between batches. Otherwise, the flour and fat from the previous batch will prevent proper whipping. The cakes can then be frozen. Because they are always wetted with some liquid, freezing does not damage the texture.
For those unlucky people (or their relatives): note that pan di Spagna can be made gluten-free by using potato starch only. With suitable creams (that is, creams thickened with starches other than flour), a pan di Spagna can thus be the base for gluten free cakes. I also love pan di spagna made with chestnut flour, which cannot really be used with most creams, but tastes wonderful on its own.
Preheat the oven to 375. Cover the bottom of a (9in) round baking pan with wax paper, and cover with butter and sugar the sides (or the whole thing instead of using wax paper). You can buy pre-cut round wax paper bases in specialized kitchen stores (eg Sur la table).
In a big bowl, put the eggs (both whites and yolks), the sugar, the salt or cream of tartar, and the optional flavors (but not butter) as desired. (They are not necessary if you plan to fill the cake with creams. But if you want to eat it on its own, some may be good. I usually use only honey.) As suggested in many cookbooks, warm eggs incorporate more air. Some recipes suggest whipping the eggs in a double boiler, but this process is quite complicated. Warming the eggs up a little by immersing them in hot tap water for a few minutes should do too. In any case, the cake will rise also if the eggs are straight out of the refrigerator, provided one whips them up long enough.
Mix at high speed for something between 5 and 15 minutes (this is where a standing mixer comes in handy.) The mixture should more than double in volume and become very soft and airy. Do not be afraid to mix too much. Most recipes say one should mix for 6 minutes or so, but that may be true of the most powerful mixers only. If I mix only for 6 minutes there's a good chance that the cake will not rise. So it may be a good idea to start with 15', and then try decreasing the amount of time after you are confident enough in preparing the cake.
After you are done, start sifting the flour and the potato starch on the mixture, very slowly, mixing it. It is essential that you sift the flour on the mixture, not into the mixture. Do so a little at a time. Mix just enough to incorporate the flour in the mixture, but do not overmix. This is the most difficult part of the preparation. Do not, I repeat, do not pour all the flour at once. If you pour too much flour and it sinks to the bottom of the pan, the batter is lost and there is no way to save it. If this happens, throw away and start again (not a big problem since the ingredients are very cheap.) Regarding the flour, you can use all purpose as well. Low protein and starch should make the cake softer. But gluten in flour helps absorb liquids, so with more flour you can soak it with more syrup. So as usual it depends on your taste.
If you want to add the butter as well, add the melted butter at the end, mixing slowly and very briefly. Otherwise the mixture will deflate and the batter is lost. Mixing butter is very tricky. It does make the cake moister, but the risk of ruining it is big. You should not even think about trying this before you master the basic pan di Spagna. I never add butter, since I tend to soak cakes in syrup anyway, moistness is not an issue.
Pour the mixture in the baking pan and put it immediately in the preheated oven. The temperature and the time depend on the oven. 350 should be good. I also used 375 degrees in an oven whose heat came from the bottom, and put the cake in the upper rack of the oven with an (empty) cookie sheet on the bottom rack, to protect the cake from overheating. Different ovens may require different treatments. Bake for approx 40 minutes, or until a toothpick comes out clean. Baking times also vary depending on the oven (and even the day, the humidity etc.), so make sure the top of the cake is not getting too dark. When ready, take it out of the oven, and carefully take it out of the pan, being careful not to break it. (You may need to slide a knife or the like in the inside of the pan.) If you use wax paper, remove it while the cake is warm.
Let the cake cool completely on a rack before using. (A rack is very useful, if you just put the cake on a dish, the humidity will make the bottom of the cake wet. If you don't use a rack, make sure that at least you turn the cake from time to time.) In my previous oven, the top of the cake tended to remain wet and sticky, and I usually preferred to peel it off (it comes off pretty nicely). The problem does not arise in most other types of oven.
If you wrap it tightly in plastic (after it has completely cooled), you can freeze it and keep it in the freezer for (more than) a month. Just bring it back to room temperature when needed. Since the cake freezes well, I usually prepare many pan di Spagna all at once, because of economies of scale (but do clean the bowl and the mixer between batches. The presence of grease may prevent eggs from beating properly). Of course, you should have many baking pans to make several pan di spagna all at once.
Note that cutting pan di spagna requires some attention (see specific cakes for more suggestions). You should cut them when they're cold, and it is even easier to cut them when they're chilled. Thus, if you freeze them, cut them before they are completely thawed.
Note also that the cake can be made with potato starch only instead of flour and potato starch. The result is more fluffy, but it is less able to absorb liquids (ie alcoholic syrups). However, potato starch makes it gluten free (for those unlucky people who have celiac disease). Creams can also be made with potato startch instead of flour. This way, you can more or less prepare most types of Italian-style layer cakes - gluten free. A big improvement on the usual gluten free cake substitutes straight out of horror movies.