I did respond to Amalia's comment (see comment on 1-2-3-4 cake) shortly after it was posted via e-mail, but I totally forgot to post it here for posterity reasons. Here's my educational input for the day...
When modifying any of Rose's butter cake recipes for high altitude, you will need to adjust a few things. Note that the sponge cake recipes in the book are a whole different animal from the butter cakes and will need to be treated differently. I will attempt to address the sponge cake in future postings.
Certainly, leavening agents need to be paired down. I usually nix any baking soda in the recipe, unless it's the sole leavener and the batter has a lot of acidic ingredients (such as a combination of mayo and natural cocoa powder). Whether you're using baking soda or baking powder, these will need to be decreased in 1/4 tsp. intervals until you find the right amount to use--and the only way you can do this is by trial and error.
I never touch the sugar amounts in Rose's recipes, as she very clearly explains in TCB that the sugar levels are at bare minimums in her recipes and decreasing the sugar will negatively affect the taste and the texture of the cake, so leave the sugar alone!
I do usually add more flour, starting with a tablespoon the first time around and going from there, and unless the recipe has a high amount of liquid ingredients, a tablespoon of flour usually works.
You will also need to add 1- 2 oz of extra egg. This can be either: 1 whole egg, 1 or 2 egg yolks, or 1 or 2 egg whites, depending if the recipe calls for whole eggs, yolks, or whites.
Lastly, you may need to add a couple tablespoons of extra liquid. One to two tablespoons usually works, but if the cake still seems a bit dry, you can add a bit more.
One note about liquids: if you experience problems with the cake setting up quickly before it falls, consider using a acidified liquid. You can do this by replacing a few tablespoons of the milk with lemon juice or vinegar, or replacing the milk altogether with buttermilk (with a full fat buttermilk if you can find it). If you have no buttermilk, you can try using a full fat plain youghurt thinned with a bit of milk or full fat sour cream, also thinned--add a bit of vinegar to either the yoghurt or sour cream to help it acidify. Allow time for the youghurt/sour cream/milk mixture to acidify fully (about 10 minutes). If you do use buttermilk, youghurt, or sour cream, keep in mind that flavor profile won't be the same, but it may be necessary if you really want to use the recipe.
About chocolate and cocoa powder:
If you are using melted chocolate in a recipe, usually no adjustments are needed in the amounts, as melted chocolate is slighty acidic and works well in high altitude cakes.
If you are adjusting a chocolate cake recipe and it calls for Dutch processed cocoa, you may need to replace either a portion or all of the Dutched cocoa with natural cocoa. The reason is that Dutched cocoa is alkalized (meaning its acidity is neuralized to bring out more chocolate flavor and make it less biting), but this also lowers the PH level of the batter, and at high altitudes, can prevent the cake from setting up before it collaspes in the oven. I would probably try using buttermilk in the recipe first, before swapping out the Dutched cocoa.
So there you have it: a list of useful pointers when you are attempting to adapt butter cake recipes (especially Rose's!) for high altitude.
Remember, a successful high altitude cake is only accomplished through trial and error!
I will have future postings on some of the cakes in The Cake Bible and what I did to modify them for my altitude.