GFBaker writes in the comments of an article on baking soda and baking powder:
I have a question I desperately need some help with, and soon! I am doing a gluten-free chocolate cake for my brother's wedding in 10 days. I've converted our favorite old chocolate cake recipe to gluten-free quite successfully, which is a challenge in and of itself. However, I wanted the cake to rise just a tad bit more, so I added a 1/2 tsp more baking soda to the recipe, and it actually turned out more dense. Did I take it too far? Should I have added a bit of baking powder instead? Or might I have had my water too hot, or something else? Here are the proportions of flour, liquids,and acids, without the addition of the extra 1/2 tsp baking soda:
3 cups GF flour blend (superfine brown rice flour, potato starch and tapioca flour)
3/4 cup natural cocoa
1-1/2 cup sour cream
1 tablespoon baking soda
3/4 cup hot water
I would love some help - I don't want to do multiple stab-in-the-dark trial batches this week to get it right. Thanks!
In your regular, gluten-filled cooking, there is a rule of thumb th at says that you don't want more than about 1 teaspoon of baking powder or 1/4 teaspoon of baking soda for a cup of flour. The reason for this is that the batter can only hold so many bubbles made from the chemical leavening before they are able to break through the surface, and once that happens, it is really easy for a whole mess of other bubbles to break through as well. This is probably what happened when you tried to increase the amount of baking soda. If it just didn't get any higher, I would suspect that you needed baking powder instead, but making it worse means you added too much.
Unfortunately, I don't do much gluten-free baking, and I don't know the proper formula for baking soda to the combination of flours that you used above. Fear not, because there are still things you can do to increase your cake's rise without resorting to drastic measures.
If you had a chance to do some extra cakes, I would actually suggest trying a little more baking soda or powder, but being very careful to mix the dry ingredients thoroughly before combining with the various wet ingredients. At some point you may give that a go if you weren't thorough with the mixing, because if the bubbles are evenly distributed, you have a bit more leeway before they break through. Still, that takes experimentation, and we don't want to deal with that right now.
Instead, I recommend taking advantage of the six eggs. If you're not doing so, separate out the eggs and the whites. However you're meant to use the eggs, do so with the yolks, but save the whites and whip to soft peaks. When the rest of your cake is combined, Take 1/3 of your whites and mix in to the cake, then take the rest and fold it gently in. This will, hopefully, give you a lot more lift to your cake without sacrificing the structural integrity when you are baking.
Now, the ingredients you gave me seem incomplete (I say that because there's no sugar, and that seems unlikely to me, but I make allowances for things that are out of my experience), and I don't know the method to make this cake, so you may be doing some of this already. Essentially, I'm suggesting making the cake more of a sponge cake than whichever method you're currently using.
As you have a few days, send me an email at firstname.lastname@example.org and let me know if you're missing any ingredients in the list as well as which method you're using. After we chat a bit, I'll revisit this topic with the updates.