The pans are 9 1/2 inches. You may be on to something there, the recipe calls for 9 inches. I guess that could have accounted for 1/2 inch less in height.
My baking powder is fairly new, but my baking soda - I don't remember when I bought it. Maybe it's time to replace it.
Here's the finished product:
3layerCake.jpg (23.9 KiB) Viewed 2548 times
In some ways, it's good that I had to add an extra layer - I couldn't have fitted all the chocolate-hazelnut filling between just two layers. You can't see it from the angle, but there's a good amount of filling between the bottom two layers too.
I didn't use cake flour; I specifically looked for a recipe that called for all-purpose, since I didn't have any on hand.