Hannigan and partner Sasha Farber have survived 10 weeks of competition and are headed to the finals on Dec. 5.
Talking with ET's Denny Directo from the ballroom floor after the semi-finals, Hannigan, 49, spoke about how she has changed week after week since beginning the show.
"I mean it's insane," the Buffy the Vampire Slayer star said. "I can't even believe -- he keeps saying 'Body by Sasha.'"
"Contact me on social media to make you look like this," joked Farber, 39, showing off Hannigan's figure in a long blue gown from their waltz.
With the increase in activity, however, she says her "healthy eating" has gone out the window as she is mostly focused on just replenishing calories burned from hours of dancing each day.
"The other day in the studio -- pizza, pizza, pizza, pizza," Farber said of what they were eating while rehearsing.
"Yeah, the PT [physical therapist] said 'oh you should have milk and bananas before bed for calcium and potassium' and we were out of milk so we had ice cream," Hannigan joked of her new bedtime routine.
The actress has come a long way since almost not being able to perform during week 3 after falling ill just hours before the show. At the time, she revealed that she was so sick she could barely get out of bed and needed a barrage of electrolytes to hit the dancefloor.
While she has continued to improve week after week, she told ET she is still facing major struggles when it comes to learning and perfecting the dances.
"This week was so hard for me," Hannigan said. "My body's falling apart, my knee wants to leave my leg and the jive is just a mean dance. It's really awful, it's awful."
"I love the jive," Farber responded.
"Only deranged people love the jive," the How I Met Your Mother actress retorted.
The pair pulled off a score of 25 out of 30 with their jive to "Footloose" by Kenny Loggins before netting a 26 out of 30 for their waltz to "Come Away With Me" by Norah Jones.
It's now onto the finals for the pair who will face off against an unprecedented four other stars -- Xochitl Gomez, Ariana Madix, Jason Mraz and Charity Lawson -- to win the coveted Mirrorball Trophy.
The Dancing With the Stars season 32 finale airs on ABC and Disney+ on Tuesday, Dec. 5 at 8 p.m. ET.
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