I didn't eat at the original branch of El Cardenal, but rather the one in the Sheraton near Alameda Park, as it was an easier walk from my hotel on my last morning in town. I think I arrived at about 8:30 on a Sunday and the place was already half full. When I left about an hour later there was a line out the door. The restaurant gets great reviews in general, but they're especially famous for their breakfasts, especially the conchas con nata.
The concha is the quintessential Mexican breakfast pastry, a light sweet roll in the shape of a shell and covered with sugar. It's not overly sweet, and it has a fluffy consistency somewhere between bready and cakey. El Cardenal is considered by many to have the best conchas in the city (though some give the honor to a place called Bondy, in the upscale Polanco neighborhood). A concha is good on its own, but the icing on the cake is nata, which you order separately. It's actually the cream that forms on the top of milk when it's heated, though it's served cold. I guess there's a secret to making it just right without scalding. El Cardenal was my only nata experience, but it was positively culinary-erotic--an amazing combination of freshness, richness, and a sly come-hither sweetness that didn't even hint at cloying. An order of nata as served at El Cardenal is probably sufficient to cover about three conchas; I slathered mine on a concha and another sweet bread, this one bow-tie shaped.
The concha was just the beginning. I also had another house specialty, the tortilla con escamoles--an omelet with ant eggs. You heard me, ant eggs. And they're much tastier than grasshoppers. Escamoles are considered a delicacy in Mexico, and they're quite expensive. I believe they're most often served mixed into butter to spread on corn tortillas (Restaurante Chon, where I ate my chapulines, serves them that way).
The escamoles have a subtle, nutty flavor, nothing like what I'd imagine a "formic" taste to be. They look like little off-white capsules or large rice grains (if you click on the photo above you can see a few wayward ones on the left of the plate--don't confuse them with the cotijo cheese on the beans).
The other memorable breakfast was at a restaurant so legendary that a Mexican rock band took its name for their own--Cafe de Tacuba, established in 1912. It's right in the historic center, and once settled in the vintage interior you'll feel as if you've traveled back in time. Like El Cardenal, Cafe de Tacuba serves three meals a day, which is not that common in Mexico City. Locals love the place and it seems to have maintained the quality of its food all these years. The waiters have an endearing dual nature of attentive and friendly yet somehow bored or pissed off, so as a New Yorker I felt right at home. And the waitresses wear these wonderful white-as-snow uniforms with headgear that makes them look like a cross between nurses, nuns and maids. At Cafe de Tacuba I had huevos con mole and a hot chocolate (Mexicans make so-so coffee but great chocolate), which is served with semi-dry biscuits.