theme explanation: Blossom in 59A, Bubbles in 16A, and Buttercup in 38A make up the Powerpuff Girls as hinted by this puzzle's title and clue for 66A... Bliss (introduced in the rebooted version) makes a cameo at 1A
Of course I got the BUTTERCUP part, realized that the theme was about the Powerpuff Girls, and then completely blanked on the names of the other two. Typical. (And I say "realized" because 66A was the very last clue I saw)
Coercing into helping (hint: the base verb is a mythical fire-breathing creature with one of its letters repeated)
15A
"At Seventeen" singer Janis
16A
Don Ho song that a contestant on "The Mole" was subjected to listen to all night long at Anderson Cooper's "funhouse" (2 words)
18A
Marvel shapeshifting supervillain and leader of the Deviants
19A
"How's it goin'?"
20A
Like some martinis
21A
Bad-mouth
23A
There are eight in a gallon: Abbr.
25A
Wallach of "The Good, the Bad and the Ugly"
27A
___ fire under (roused to action) (2 words)
28A
Apple device discontinued last year
30A
Pizza-like food purveyed by D.P. Dough
33A
Programming language that I misread as "C Number" because I'm a dumb-dumb (musical clue: D-flat equivalent)
36A
Member of a South Asian diaspora
37A
Shares again on Twitter, briefly
38A
There's no wrong way to eat one, according to Reese's (3 words)
41A
When you sing "The ABC Song" these are the first 3 of the 5 letters you sing during the fast part
42A
Parting word that I like to use because it sounds like my last name
43A
Investor Warren Buffett, by birth, as well as civil rights activist Malcolm X, dancer Fred Astaire, president Gerald Ford, and my friend Dee who is oddly obsessed with the seal emoji
44A
Bums out
46A
Booby ___ (hidden danger)
47A
Begged
48A
Fish found in Japanese gardens
50A
Paramedic in an ambulance: Abbr.
52A
Baby angel
54A
Texter's "Holy moly!"
56A
Motorist's 180° turn, slangily (here's another hint: Chinese takeout stir-fried dish "chop ___" but minus the "s")
58A
Thomas who co-wrote and sang the Grammy-winning hit "Smooth" by Santana
59A
National flower of Taiwan featured on its Olympic flag (2 words)
63A
British singer Rita who released "Carry On" with Norwegian producer Kygo, as a standalone single for "Pokemon: Detective Pikachu"
64A
Not guaranteed, like the coming of tomorrow
65A
According to
66A
Extra ingredient that Professor Utonium accidentally added to his concoction that turned the perfect little girls into the Powerpuff Girls (who are hiding in this puzzle) (2 words)
Verticales
1D
West Coast summer clock setting: Abbr.
2D
"Exodus" novelist Leon
3D
Amassed, as a bar tab (2 words)
4D
Like kushari (cumin scented lentils and rice topped with pasta, a spicy, garlicky, vinegary tomato sauce, and crispy fried onions) and basbousa (syrup-soaked semolina cake sweetened with orange flower water or rose water)
5D
Move up and down repeatedly
6D
Musician's forte?
7D
Born from or produced by animals, plants, or people that are closely related
8D
Ancient prophetess
9D
Show whose first guest host was George Carlin, in 1975: Abbr.
10D
It gets snuffed out for an eliminated contestant on "Survivor" (the tribe has spoken) (2 words)
11D
Rabelaisian giant with an enormous appetite (whose name is now associated with something massive in size)
12D
Privy to (2 words)
14D
Beryl or tourmaline
17D
Big name in student loans (2 words)
22D
Fashion designer Claiborne
24D
Refreshed, as decor (2 words)
26D
"Law & Order: SVU" actor whose stage name sounds like a drink
29D
Join voluntarily (2 words)
31D
Fur tycoon John Jacob
32D
"Pardon the Interruption" channel
33D
They report to sergeants in the Marines: Abbr.
34D
Flag signalling system that the album cover for the Beatles' 1965 album Help! was originally intended to use by portraying the four band members spelling "help"... however, the result was deemed aesthetically unpleasing, and their arms were instead positioned in a meaningless but aesthetically pleasing arrangement
35D
Mustache style
36D
Bonnie and Clyde, for example
39D
Take pleasure, as in one's glory
40D
Subject of much of Jay-Z's "Decoded" (2 words)
45D
Architect Mies van ___ Rohe whose motto was "Less is more"
46D
Quality of a musical sound
49D
That little extra je ne sais quoi
51D
Thompson who plays Valkyrie in Marvel movies
52D
___ top (garment that exposes the midriff)
53D
Macaw protagonist of the 2011 animated comedy "Rio"
55D
Steal, slangily
57D
Cuban Olympic medalist wrestler Romero nicknamed "Soldier of God"
60D
Chapel Hill school who alongside Harvard was involved in a landmark decision of the US Supreme Court in which the court ruled that affirmative action programs in college admissions processes are unconstitutional
61D
Jamaican singer with the 2015 #1 hit "Cheerleader"
62D
Acura model whose alphanumeric moniker stands for "Multi-Dimensional" luxury (hint: 1510 in Roman numerals)