Maria Rojas serves her mollejas (sweetbreads) on two griddle-heated corn tortillas with caramelized onion and chopped cilantro on top. She puts the sweetbread tacos and a half of a lime on a Styrofoam plate and sets out a little stone molcajete full of dark hot sauce beside it. The sweetbreads have a delightfully mild flavor and a lovely fluffy texture that might remind you of the filling of a German veal sausage, like bratwurst or weisswurst. The tartness of the lime juice and sweetness of the onion are beautiful complements. And that dark brown salsa will make you weak in the knees. The chocolate-colored salsa looks like it's going to be hellishly hot but, in fact, it's astonishingly rich and mellow. It's made of dried cascabels that have been soaked until soft, pureed and lightly seasoned with salt, pepper and a touch of garlic — just the thing on a sweetbread taco.