In the sections that follow, the soprano is asked to wobble vibrato like bird song; dash through rhythmically complex runs that jump throughout the staff; laugh deeply, as if possessed; play percussion and clap; and navigate a page-long cadenza in which each measure is a minefield of intonation and technique. Yet between these passages is also astonishing beauty, even the sublime: rending melodies fit for lullabies, ethereal humming that hovers delicately, like a visiting spirit.

“Most of my work involves pushing the envelope of technical mastery,” Mr. Zorn said. “For me, challenges are opportunities. You must believe enough in what you are doing to put yourself in harm’s way — because only through discipline and extremes of experience is one able to transcend the trivial and mundane. As a sacred piece, ‘Jumalattaret’ needed very much to capture those feeling tones.”

Ms. Hannigan and Mr. Gosling began rehearsing the piece, with help from Mr. Zorn, in late 2017, with the intention of recording it as soon as possible. But it wasn’t coming together. The score doesn’t offer many opportunities for the singer get her pitch from the piano, so Ms. Hannigan needed to ingrain the part into her muscle memory. To master the breakneck speed, she began by learning the notes at a slow hum to avoid damaging her voice.

“You’ve got to tame the wild horse and get a saddle on it,” she said. “This piece took a lot for me to be able to do that.”

Among the most grueling sections were the ones in which Ms. Hannigan and Mr. Gosling were required to remain together despite very different virtuosic lines. “In the second movement,” Mr. Gosling said, “it’s quite a brief passage, but it’s polyrhythmic and it goes all over the keyboard. But then we have to suddenly realign. You have to be selectively deaf.”

They decided to postpone the recording. (A session still hasn’t happened, though Ms. Hannigan feels she is ready now.) Instead, they gave themselves nine months to prepare for what would end up being the piece’s premiere, at the Jazz em Agosto festival in Lisbon. In between, Ms. Hannigan joined her family for a summer vacation in Nova Scotia.