Worcestershire were on the receiving end of Tom Westley's maiden double hundred as Essex built a substantial first innings lead on day two of the Specsavers County Championship match at Chelmsford.
Ed Barnard, Jack Shantry and Joe Leach all enjoyed one success with the ball after Essex resumed on 111-1 from 27 overs in reply to Worcestershire's 230 all out.
But Westley was unmoved in going to an undefeated 238 off 361 balls with 37 boundaries out of 512-4 from 123 overs.
Miguel Cummins induced a false stroke out of Alistair Cook in his first over of the day, an edge which flew past the slips for four.
Cook brought up his half century with a boundary off the West Indian paceman from 81 balls and nine fours.
Barnard replaced Leach and he struck in his first over of the day with the prized wicket of England Test skipper Cook for 66.
He nicked a delivery from the England Under-19 all-rounder and Ross Whiteley at first slip held onto the chance at the second attempt.
Cook's stand was worth 153 in 37.5 overs with Westley.
Barnard could have had a second success when Westley on 85 survived a difficult head high chance to skipper Daryl Mitchell at second slip.
A six by Bopara off George Rhodes brought up the 200 in the 57th over.
Westley was unbeaten on 98 out of 205-2 off 58 overs at lunch and went to three figures with an extra cover drive for four off Jack Shantry.
His hundred came off 182 balls and contained 18 boundaries. Ravi Bopara cut Cummins through backward point to reach an 89 ball half century with nine fours and a six.
Brett D'Oliveira came into the attack for the first time in the 73rd over. He was only able to bowl three deliveries and George Rhodes completed the over although the leg spinner continued to field on the boundary.
The third wicket stand was worth 213 in 55.5 overs before Worcestershire struck twice in as many overs.
Bopara was one short of his century when he edged a Shantry delivery to give Whiteley his second catch of the day.
The former England pkayer hit one six and 15 boundaries in his 172 ball knock.
Then in the next over Leach enjoyed a second success of the innings as Dan Lawrence (1) was pouched at backward point of a ball short of a length to leave Essex on 368-4 in the 97th over.
Westley went past his previous highest score of 185 and went to his double hundred with four through mid wicket off Cummins from 319 bals with 33 fours.
Essex skipper Ryan ten Doeschate (77 not out) helped Westley add an unbroken 144 in 26.2 overs by the close.
Leach finished with 2-88 from 24 overs, Barnard 1-94 from 22 overs and Shantry 1-101 from 28 overs.