Sisters

Advertising salesman Philip Woode (Lisle Wilson) is the victim of a prank on a Candid Camera-style television show and wins a dinner for two at a New York restaurant. He takes with him Danielle Breton (Margot Kidder), a naive young Canadian model and aspiring actress who was hired as part of the prank. At dinner, they are interrupted by Danielle's controlling ex-husband Emil (William Finley), but Philip has him thrown out by security. They arrive to Danielle's Staten Island apartment, but spot Emil waiting outside; Philip pretends to leave to lure him away, then returns through the back entrance and the two make love on the living room sofa.
The next morning, Danielle wakes and feels unwell. Philip overhears an argument between her and her sister in the bedroom, and accidentally knocks Danielle's pills down a drain. Danielle explains to Philip that her twin sister Dominique had arrived to visit because it is their birthday, and also asks him to go to the drug store to pick up some more pills. He does so, and also buys a birthday cake for the two. When he returns, he is suddenly stabbed to death by Dominique, who is crazed and delirious.
The murder is witnessed by Grace Collier (Jennifer Salt), a reporter who lives in an apartment across the way, who calls the police. Emil knocks on Danielle's door and finds the scene, then helps her clean up the blood and hide Philip's body in the retractable couch. Two detectives arrive but are unhelpful and resentful toward Grace because of an article she had previously written about police brutality. Grace accompanies them on a search through Danielle's apartment, who testifies that she had been alone all morning and the previous night; they find no evidence, while Grace finds the birthday cake with Danielle's and Dominique's names written on it, but trips and destroys the cake before she can show it to the detectives.
Grace is convinced that Danielle is hiding the murderer, and convinces her editor to let her investigate the story. He agrees, but forces her to take along Joseph Larch (Charles Durning), a private investigator. He searches Danielle's apartment under the guise of a window cleaner, and is convinced that the body is hidden inside the couch due to its unnatural weight, but is interrupted by moving men who are directed to remove the couch. Larch follows the moving truck, while Grace discovers that Danielle and Dominique Blanchion were Canada's first conjoined twins, who were separated only recently. She is also told that Dominique died during the operation. Grace then tails Emil and Danielle to a mental hospital and witnesses him subdue and sedate her, but is caught. Emil, a doctor at the hospital, convinces the staff that Grace is a new patient and has her locked in a room where he hypnotises her, telling her that the murder did not happen.
Grace wakes from a bizarre dream to find Emil confronting Danielle. It is revealed that after Dominique's death, Danielle began to suffer split personality disorder, with Dominique representing her murderous side whenever a man showed affection toward her. Danielle slashes him with a scalpel, and the two fall back on Grace, who awakens and screams as Emil dies of blood loss. The police arrest Danielle, who still denies that she has committed murder or that she has a sister.
Later, Grace is interviewed by a detective about the death of Philip Woode, but still under Emil's hypnosis, she completely denies her previous accusations. The final scene shows Larch, watching the sofa at a derelict train station in Canada while perched on a telephone pole.

Question: Who destroys the cake ?
Answer: Grace destroys the cake.