A Mafia turncoat shone a floodlight on the infamous Lufthansa cargo heist Tuesday, telling rapt jurors in detail about the planning, execution and short-lived euphoria of the wiseguys who pulled off one of the biggest robberies in American history.

Testifying at the trial of his cousin, Bonanno capo Vincent Asaro, Gaspare Valenti recalled the moment when he and his gang of thieves breached the Lufthansa hangar at Kennedy Airport in 1978 and found boxes crammed with cash and jewels worth more than $6 million.

Gangster Tommy DeSimone, played by Joe Pesci​ in “Goodfellas,” the 1990 Martin Scorsese mob movie that made the crime legendary, watched the Styrofoam peanuts pour out as he reached into one of the boxes and hit pay dirt.

“This is it! This is it!” DeSimone yelped after yanking out bundles of cash totaling $125,000 apiece, Valenti, 68, told a packed Brooklyn federal courtroom.

Prosecutors are hoping to close the door on the Lufthansa case — which has bedeviled law enforcement for decades ​— through a conviction against Asaro, the first person ever tried for the crime. He is charged with the Lufthansa heist along with a raft of other mob crimes — including a 1969 murder — under a federal racketeering indictment.

Valenti’s disapproving son, reputed Bonanno associate Anthony “Fat Sammy” Valenti, glared at his turncoat father from the gallery as the mob rat spilled the beans on the famous heist.

He first learned about the plan from his cousin Asaro, Valenti said, who mentioned that Luchese associate James “Jimmy the Gent” Burke — played by Robert De Niro in “Goodfellas” — had something cooking.

“He said that Jimmy Burke has a big score at the airport coming up,” Valenti said. “And you’re invited to go.”

As the plan coalesced, the crew regularly convened at Burke’s Queens dive bar, Robert’s Lounge, to discuss strategy for what they thought would be at best a $2 million haul.

In addition to Burke, DeSimone and Asaro, the raspy-voiced gangster listed the other members of the Lufthansa platoon — Angelo Sepe, Joseph “Joe Buddha” Manri, Danny Rizzo, Louis Cafora and Burke’s son, Frankie Burke.

Fearful of capture and a potential life sentence after two priors, Burke told his collaborators he couldn’t enter the building and would instead park nearby at 150th Street and Conduit Avenue with Asaro, Valenti said.

“Make sure you do everything you’re supposed to do,” Asaro instructed his younger cousin. “Don’t dog it. Don’t run away. Stand your ground.”

Armed with architectural plans of the area provided by Marty Krugman​ — presented as Morris Kessler (of Morrie’s Wigs) in “Goodfellas” ​— the crew met up at Burke’s house the night of the robbery, Valenti testified. “All in black,” he said of their clothing. “Ski masks. Gloves.”

Apparently champing at the bit, DeSimone​ ​ told Valenti in the hours before the job to accompany him to a nearby marsh so he could test a new silencer. “He had a nickname. ‘Tommy Two Guns,’ they used to call him,” Valenti recalled. The witness said he was leery of accompanying DeSimone for the firearms demonstration. “He was known as a killer,” he said. The silencer test passed without incident.

Valenti cut the bolt to a gate at the hangar so the crew could drive their black Ford van to the front of the facility, he testified.

As Valenti cut the lock, the van with the rest of the crew pulled around to another side of the hangar and let them out, he said.

“He must have surmised something was up, and he started running towards the door of the building and we chased him,” Valenti said. “I hit him with the pistol in the head. He yelled, ‘Help!’ ” Burke then stuck a gun in his eye and told him to get into the van. “Just relax,” Valenti recalled telling him. “Nobody’s going to get hurt.”

Burke, they were told, would call in bomb threat to the opposite side of the hangar to divert law enforcement attention during the operation.

With the bolt cut and the gate open, Valenti and Frankie Burke – Jimmy’s son – were able to drive the van onto the grounds and near the hangar to await their accomplices, Valenti testified.

But a guard spotted the suspicious looking vehicle and approached the two men.

“He must have surmised something was up and he started running towards the door of the building and we chased him,” Valenti said. “I hit him with the pistol in the head. He yelled ‘help!”

After one of the staffers opened up a vault and DeSimone made his joyous discovery, the crew formed a chain and loaded up countless boxes of cash and jewels into the vehicle and sped off.

“We loaded 50 boxes,” he said. “There were burlap sacks of gold chains, crates of watches, metal boxes with three drawers in them — and each drawer had diamonds and emeralds in it. All different stones. And we loaded everything into the van.”

To Valenti’s chagrin, it was decided the loot would be dumped in the basement of his family home at 2 a.m. as his wife, kids, mother and sister slept upstairs.

With the haul off the street, the triumphant gang sorted through the treasure for two hours. “I was separating gold chains and watches and diamonds and emeralds and rubies and looking in the drawers,” Valenti recalled.

​​After locking up all the windows and doors to what had suddenly become Brooklyn’s Fort Knox, the group met up at one of their dives — Fat Andy’s on Liberty Avenue — to take stock of their historic score.

“It was euphoria,” he said. “We thought there was $2 million in cash, and there was $6 million. Without the gold. Without the German money.”

But amid the celebration, members of the crew immediately realized they were now under intense law-enforcement scrutiny. The warnings began to circulate​ in a manner familiar to any fan of “Goodfellas.”

​”Don’t spend anything,” Valenti recalled hearing. “Don’t buy anything major.” Asaro also warned other mobsters could come gunning for them and their riches.

But the advice was not heeded. Some who received cuts opened up an Ozone Park nightclub called Afters — for “after Lufthansa.” The disco was hot enough to host performances from Blondie and Gladys Knight, Valenti recalled.

Asaro bought himself a house in Moriches, LI, as well as a brand-new Lincoln sedan — Bill Blass edition, Valenti recalled.