The Italian, first in the season-opening downhill at Lake Louise, now heads the overall standings on 310 points.
The Croatian Janica Kostelic is second eight points behind, but Kostner's progress in giant slaloms this season makes her a favourite for World Cup supremacy.
"I know I have a reasonable chance to win the World Cup this season, especially given my improvement in giant slaloms.
"Giant slalom is still my third event, but it's a very nice feeling to know you have a chance at overall victory," she said.
The super-giant slalom world champion in 1996 and 1997, Kostner had surprisingly achieved only one prior World Cup success in the discipline.
"It was about time I won a second because it's a discipline I like very much, almost as much as the downhill."
The victory came in the most unexpected place as she confessed her dislike of Val d'Isere's piste.
"I used to struggle because it's so tight but it went fine this time. It's a reward for all the fine work I did over the summer," added Kostner, the first Italian to win a speed event on the French slope.
The German Hilde Gerg, winner of a super-G here two years ago, finished second for the third successive time this season. Gerg, runner-up to Kostner in Lake Louise by 0.08sec and edged out by 0.02 in the super-G at the Canadian resort by the Slovenian Mojca Suhadolc, missed out this time by 0.07.
"The good news is that I'm consistent," said the German.
"But my aim is to win the World Cup and I Iost 60 points in three races for just a dozen hundredths of a second. It may prove costly at the end of the season."
The veteran Swede Pernilla Wiberg impressed by finishing third, her first podium placing this season.
"I wanted to do something great here because I had been skiing like a grandma so far this season," said the multiple world and Olympic champion.
Wiberg, 29, is the only skier still in action with wins in all five Alpine disciplines, but she has been prone to injury.
"Everything's fine this season. I'm in great shape and my boyfriend is with me on the tour," she said.
Wiberg will again be among the favourites in today's giant slalom which takes place on the same piste.Reuse content