After conceding an equaliser and then being forced to play the final 20 minutes with a player fewer, Greg Lincoln's troops survived a penalty, weathered relentless pressure and then struck twice in stoppage-time to seal an extraordinary 3-1 victory at the Altair Group Community Stadium.
The Hammers were the only academy side in the competition to progress from the group stage, advancing to the showpiece - where they will face Boreham Wood - via knockout victories over Boston United and Tamworth.
U21s lead coach Greg Lincoln described the occasion as exactly the type of test Academy football is designed to provide.
Lincoln said: “I think it was a game that had absolutely everything. You enter these competitions to be exposed to senior football, and you saw that tonight. The crowd had a huge influence on the game as well.”