Essex overcame a mid-innings crisis to book a date at Trent Bridge next month in their quest for the Twenty20 Cup.
They did so with four deliveries to spare thanks to an unbroken partnership of 76 involving Ryan ten Doeschate and James Foster.
They came together with the total on 73 after Darren Gough, Will Jefferson and Ravinder Bopara had departed in successive overs.
At that stage the contest appeared on a knife edge but Ten Doeschate and Foster were to carry the game away from the visitors with some lofty hitting. Both had spectators in a capacity 6,000 crowd taking evasive action as they saw their side to a fifth victory in as many outings in front of their own supporters. Only briefly had Yorkshire threatened to get on top of the Essex bowling.
James Middlebrook's one over of off-spin cost 16 and led to him being withdrawn in favour of Tim Phillips. It was a decision that was to strangle the life out of Yorkshire's hopes of setting a challenging target.
The left-arm spinner conceded only 11 runs in his four overs as he varied his flight and pace intelligently.
He also picked up the wicket of Gerard Brophy, the only visiting batsman to show any fluency as he hit six boundaries while making 43 from 31 deliveries. It was Phillips who ended his resistance, bowling him as he tried to drive through the off side.
Darren Lehmann, returning to action after a calf injury, was so ill at ease that he spent 39 balls making 40. He scored only three boundaries and his frustration was apparent when he appeared to question the dismissal of Richard Dawson in the final over of the innings. There was a suggestion Ten Doeschate had stepped on the cover boundary rope to take his catch but Dawson was sent on his way after the umpires consulted with the fielder.
When Essex replied, Mark Pettini twice lifted the Australian paceman Jason Gillespie over the mid-wicket fence in the third over. But when he fell to a superb catch by Andrew Gale trying for a third, Essex lost their way before Ten Doeschate and Foster revived them.