For two 6" layers:
Prepare pans (line and grease). Preheat oven to 350F
Beat until thick:
3 egg yolks
1 tblspn sugar
1/8 tsp salt
Melt (I use the microwave, for behold, it is easy), stir thoroughly, and fold into the yolk mixture:
3 oz good dark chocolate (I used 60% cocoa Ghirardelli's chips for a good blend between "not ridiculously expensive" and "still tastes good")
1 tblspn water
Beat until somewhere between foamy and soft peaks:
3 egg whites
Add to the egg whites:
1/4 cup sugar
And beat until stiff.
Fold 1/3 of the whites into the yolk/chocolate mixture until thoroughly incorporated, then fold the rest in. Divide between the two prepared pans (trying not to deflate it too much in the process) and pop it in the oven (gently).
Bake for, if I recall correctly, something in the neighborhood of 20 minutes; refrain from jumping jacks while they're in the oven. When the top is just barely starting to brown browner, take them out and, if they're at all attached to the sides, quickly run a knife around the edge of the pan to release them. Allow to cool slightly. (at this point, they look very alarming; it's okay)
For ease of dealing with, I take a tip from smittenkitchen (where this particular version of this recipe is adapted from) and unmold them onto a lined cookie sheet (I used waxed paper). Peel off the liners, allow to cool, then pop them in the freezer until firm. This way, they don't crumble when you're trying to work with them, and it doesn't seem to negatively affect taste/texture at all. Stack with filling, fill with filling, or fail to resist the urge to eat an entire leftover 6" layer for breakfast (it's mostly eggs, right? Eggs are good for breakfast...). I thought three layers filled with whipped cream/pastry cream and frosted with whipped ganache was about the perfect height for a celebratory 6" cake; four would be unusually tall, but tasty, and two would be fine, but not very high.