What kind of bread does Calpurnia give Scout to cheer her up?

Calpurnia made "crackling" bread.

Perhaps Calpurnia sensed that my day had been a grim one: she let me watch her fix supper. “Shut your eyes and open your mouth and I’ll give you a surprise,” she said.

It was not often that she made crackling bread, she said she never had time, but with both of us at school today had been an easy one for her. She knew I loved crackling bread.