Held at Nic’s On Beverly, Trachtenberg, feted Siriano, 39, alongside fellow stars like Ashlee Simpson, Leslie Jones, Krysten Ritter and Alicia Silverstone.

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Nearly everyone dressed the part in various Chirstian Siriano creations, including Trachtenberg who wore the designer’s mid-length little black cocktail dress with delicate ruffled details, which she coupled with a Heidi Daus statement necklace strung with a door knocker lion pendant at the center.

Instead of heels, Trachtenberg opted for Tory Burch Esme flats with rhinestone-embellished bows. In photos from the event, she looked relaxed and happy, as she celebrated Siriano’s career milestone.

At the time, Trachtenberg captioned the selfie: “About last night…” as she posed in front of the oil painting, mere moments before she would be “heading off to celebrate my friend @csiriano at his 15th anniversary book party! Thank you Jay and Chris and Christian! More photos to come.”

Presley Ann/Getty Images for Christian Siriano
She captioned another pic with Simpson, 40, “Cheeky 💋,” showing the lighthearted mood of the evening.

The day after the party, Trachtenberg posted the Instagram photo in front of the “beautiful custom painted art piece,” which she noted was a birthday gift from her “love” Jay Cohen, whom she said commissioned the piece from “the amazing artist” Chris Rivers — a self-taught English drummer-turned-painter known for bold, abstract displays of brilliant color.

Trachtenberg rose to fame from a young age, memorably playing the lead in the 1996 movie Harriet the Spy at age 10, then later starring in three seasons of Buffy the Vampire Slayer before she turned 18.