Sara Bareilles and Jessie Nelson‘s 2015 musical adaptation of Adrienne Shelly‘s film, about a small-town waitress who processes her feelings via her skillful baking, brings its sugar and spice concoction to the West End. The result is a tasty treat – albeit with the odd ill-mixed ingredient.

Jenna is trapped in a loveless marriage to the brutish, controlling Earl, and thoughts of escape – encouraged by fellow Joe’s Diner servers Dawn and Becky – are dashed when she discovers she’s pregnant. But the arrival of the delectable Dr Pomatter offers a new avenue, as does her dream of winning a lucrative baking contest.

Diane Paulus (part of an all-female lead creative team) helms a vivid and charming production, with witty, inventive movement from Lorin Lattaro making clever use of an ensemble mirroring the protagonist’s feelings and of the retro diner set, with its wheeled trolleys stacked with baked goods, and mischievously misappropriated cooking utensils.

There’s a sense in which Waitress wants to have its cake and eat it too, revelling in the old-fashioned charm of this Southern community, while also undercutting it with flawed inhabitants. Some supporting characters are more cheery cartoons, and yet the show is remarkably unsentimental about marital compromise, and its romantic plot is both raunchy and, at times, thoughtfully realistic.

But there’s still something of a rushed fairy tale ending, particularly ill-suited to Jenna and Earl’s relationship; rather than getting into the psychological nuance of domestic abuse and coercion, Earl is presented as a dumb Neanderthal, and the solution is dangerously pat.

Nevertheless, there’s a refreshing candour to the writing, a care which echoes that of Jenna’s home baking, and the (still rare) pleasure of its female point of view. An amusing early number revolves around a pregnancy test, and the friendships between the three women are given wonderful prominence. Most impressive: the big take-home message is one of self-knowledge and self-respect, rather than reliance on romance.

Bareilles has the singer-songwriter’s knack of sincere emotional directness. It’s a shame, then, that the sound balance isn’t quite right here, meaning some lyrics are lost. But a steamy duet between Jenna and her gyno is irresistible, and the big 11 o’clock number, “She Used to Be Mine”, is a shiver-inducing showstopper.

Katharine McPhee delivers the latter with aplomb. She’s an endearing Jenna, shrinking into herself in practised fashion as Earl bullies her, and, in contrast, joyously released via her affair and growing confidence.

Her voice is well suited to Bareilles’s instantly inviting folk/country-infused pop-rock score, and her grounded energy is a nice contrast with the collection of nervous tics that is David Hunter‘s neurotic doc – Hunter similarly adept at both dramatic and comic elements.

Marisha Wallace is almost too much of a powerhouse as Becky, but her formidable handling of boss Cal and big, belting vocals are still thoroughly enjoyable. Stephen Leask offers strong support as the put-upon Cal, and Laura Baldwin supplies some great physical comedy as the anxious Dawn.

However, she’s eclipsed by Jack McBrayer‘s neverending conveyer belt of astonishing slapstick. The 30 Rock star, making his musical theatre debut, is an accomplished clown – and his broad playing makes more palatable the pushy, almost stalkerish qualities of Dawn’s admirer Ogie – but his singing is of the “right notes in the wrong order” (or perhaps not even the right notes) variety.

There are good turns, too, from Shaun Prendergast as crotchety diner owner Joe, and – though god knows it’s time to retire the “sassy” black supporting character trope – Kelly Agbowu is a clear audience favourite as the all-seeing Nurse Norma.

Bittersweet and beautifully presented in Paulus’s fluid production, this heartfelt show is one that West End audiences should definitely sample. Hollywood Handshakes all round.

Waitress at the Adelphi Theatre until 19 October

Video: Meet the West End cast of Waitress

Video: Sara Bareilles leads the audience in song at the opening night