I just made this brioche and I'm very pleased with it.
I'm puzzled though. Where did all of this lift come from? The dough shaped in the pan for the final proof was tiny. The formed loaf was less than an inch thick, it had me nervous. The recipe calls for a sponge made with 1/4 tsp yeast, 1 TBS sugar, 1 egg, 1/2 cup flour and 2 TBS of water. That can ferment for up to a day, but I only allowed a couple of hours. It was pretty unremarkable.
Then I sprinkled over the sponge just over 1 cup of flour, 2 TBS sugar, 1 1/4 tsp yeast and 1/2 tsp salt. That sat for an hour, then I kneaded it (for no longer than normal bread), adding 1/4 lb butter and 2 cold eggs. That rose until doubled, then I punched it down and left it in the refrigerator overnight. The next day I formed the loaf in a normal loaf pan. This is where I got nervous. I didn't think there was any way that this tiny mass of dough was even going to rise to the top of the pan. It looked like a Twinkie! A 2 hour proof did see the dough rise just to the top of the pan. 35 minutes at 350F caused it to get just massive (relative to normal bread).
How? The eggs aren't whipped, it doesn't call for a tremendous amount of sugar and it takes just over 1/2 the amount of yeast that would be found in typical sandwich bread. How did such a tiny amount of dough turn into such a big loaf?