The American was leading Veronika Velez Zuzulova by just 0.12 seconds after the day's first run but turned the screw and finished more than a second clear of the eventual runner-up.
"It was not perfect, that's for sure," Shiffrin told FIS Ski of her first run. "I was pretty stiff, so I was a little bit bummed with my skiing. "
The 21-year-old said the key to her turnaround in form was realizing she just needed to have fun.
"I pushed and it felt really good," she said. "The first run I felt tight and the second run I tried to let it fly and that's such a good feeling.
"I took some free runs before second run and tried to find a solid feeling that I could rely on.
"And then when I went out of the gate second run, I just tried to make speed on every gate and never let up, and that mentality helped me a lot."
Shiffrin is developing an aura of invincibility when it comes to the slalom -- victory on Sunday was the Olympic champion's 11th straight World Cup win in the discipline.
After adding more events to her repertoire, Shiffrin is looking for her first World Cup title and increases her lead over Lara Gut to 105 points in the overall standings.
The women will now head to Val d'Isere, France -- where the men have been competing this week -- to tackle two of Shiffrin's new events, downhill and super-G.
Henrik Kristoffersen climbed to third in the overall table thanks to a pair of blistering runs in the men's slalom.
The Norweigan dominated on the difficult Criterium de la Première Neige course to beat Marcel Hirscher, his fiercest rival in the event, by over a second.
"It's a really good feeling. With all that's happened in the last month and a half, too, this is really good," he said.
The 22-year-old had dropped out of early season races after a disagreement with his national skiing federation over a proposed sponsorship deal.
"I hope you can expect me to ski well," he continued." As long as I can do that, I'm happy. I'm a little bit relieved. It's nice to slap some plastic again."
The men travel to South Tyrol for the super-G and a downhill disciplines.