The Saints, unbeaten in Europe all season, had one hand on the trophy after storming into a 22-6 half-time lead with tries from Phil Dowson, Foden and Dylan Hartley.
There was seemingly no way back for Leinster, who had been crushed by a dominant Northampton pack. No side had ever overturned a half-time deficit of more than nine points in a Heineken Cup final.
But Leinster, inspired by a man-of-the-match performance from fly-half Jonathan Sexton, swept Northampton aside with a stunning second-half performance to lift their second European crown.
Sexton scored two tries in a personal haul of 28 points and Nathan Hines crashed over for the third try as Leinster racked up 27 unanswered points.
Northampton were left desolate on the final whistle, their players strewn about the field, some in tears. Foden had never experienced disappointment like it.
But the England full-back insisted the Saints would come back stronger next season.
"We will learn a huge lesson from this game. A lot of players haven't been involved in games such as these. Our first taste of major silverware is a bitter one," said Foden.
"We play this game to be in moments like this. We had one foot through the door.
"For Leinster to come back from that scoreline probably makes the taste of victory even sweeter, and for us that little bit more bitter.
"It builds character, and we are a good enough team to go away and build from it.
"We are a young team and these moments bring us together.
"I think we are a good enough side to repeat the performances we put in this year and rebuild and make sure we are there or thereabouts again next season."
Northampton scored early through Dowson and kept their foot on Leinster's throat until the interval.
Even when prop Brian Mujati was sin-binned, Northampton's seven-man pack won a scrum against the head and Foden scampered outside Brian O'Driscoll to score.
Hartley was driven over the line just before the interval but Northampton's fairytale then turned into a horror story with Sexton their tormentor-in-chief.
O'Driscoll reported that Sexton had been like "a man possessed" and he turned his passionate words - invoking the spirit of Liverpool's 2005 Champions League final comeback - into ruthless deed.
The Irish province made some technical changes to their scrum at half-time which negated Northampton's control of the set-piece and they attacked with renewed ferocity and intensity.
Sexton touched down twice and compiled 20 points in as many minutes as Leinster surged into a lead they would never relinquish.
"They put in an outstanding performance. Jonny Sexton was outstanding with the boot and in open play," said Foden.
"There was some great rugby played by both sides. I am pretty sure the neutral spectator would have had a ball watching it - it was a credit to rugby.
"We knew it was an 80-minute game, that's the thing about rugby. There was a momentum swing and things started to go their way."
After defeat to Leicester in the Aviva Premiership semi-final, Northampton's promising season ended without silverware.
Director of rugby Jim Mallinder put some of the second-half collapse down to fatigue but in the cold light of day he will reflect on promising signs for the future.
"We played a very tough semi-final last week and an incredibly tough final today. I am nothing but immensely proud to be part of this team," he said.
"We have learned a lot this year, not just tonight. We have shown we can compete with some of the best teams in Europe.
"These lads have now experienced being in a final. Leinster have been together a long time, playing at the top level.
"Four years ago we were in the first division. We have come a long way to where we are now. Hopefully we can stay there and we can win one of these big ones."
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