After playing only six times in the first half of the season Bednar, 27, joined Leicester on loan in November.
Roman Bednar's powerful header reinstated West Brom's two-goal cushion before James Hayter's header made for a nerve-wracking finale.
The Swede was also delighted to bring in West Brom striker Roman Bednar.
Bednar had a shot blocked and Barnett directed a header wide, while West Ham's attacking threat had virtually disappeared.
West Ham took the lead when Lucas Neill volleyed in from close-range before Roman Bednar levelled with a penalty.
Bednar sent a low drive a foot wide from 14 yards as the Czechs looked to extend their lead.
But by then West Brom could have been further ahead, with Roman Bednar a constant threat for the home side.
At the other end, Camp saved well from Bednar and the keeper then tipped over a powerful left-foot shot from Brunt.
Bednar said his firm is often getting brought into the process earlier.
FORBES: Private Equity Is Open For Business, So Where's That $20B Buyout?
The unmarked Leon Barnett put pressure on Green, who fumbled and then tripped the Baggies man, leaving Bednar to demonstrate perfect penalty technique.
However, the pressure eventually told when Rosicky's header was flicked on by substitute Roman Bednar and Hubnik diverted it past McGregor from four yards.
Bednar made five appearances for Leicester in this short time at the Walkers Stadium and says the break away from The Hawthorns came at the right time.
On-loan Arsenal striker Jay Simpson opened the scoring in the 18th minute, collecting a pass from Roman Bednar, turning and lashing in a 20-yard drive.
Bednar spun intelligently in the box and fired an effort just wide of Smith's left-hand post as Hearts threatened for the first time in the second half.
In the second half, Posh keeper Joe Lewis was called into action on a number of occasions to deny Felipe Teixeira and Bednar as Albion continued to dominate.
Bednar, who is advising NYSE Euronext on its planned merger with Deutsche Borse, also got an interesting look into what the public thinks of that deal before the panel began.
FORBES: Private Equity Is Open For Business, So Where's That $20B Buyout?
The panel, which included Andrew Bednar from investment bank Perella Weinberg as well, agreed that the only limit on the size of buyouts is the depth of debt markets.
FORBES: Private Equity Is Open For Business, So Where's That $20B Buyout?
Given did brilliantly to keep out a Bednar effort with a one-handed save and his stop to keep out a Giles Barnes header from Somen Tchoyi's cross was even better.
Midfielder Graham Dorrans was looking an assured figure for Di Matteo's men and he produced a delightful ball to find an unmarked Bednar, whose header was tipped over by Logan.
West Brom's early dominance was almost rewarded when Olsson headed against the bar after Valero's shot was well saved by Boaz Myhill, and Roman Bednar almost squeezed in the second rebound.
And it was only Anthony Gerrard's bravery that came between Bednar and a clear chance as the Bluebirds centre-back dived in to incept a pass bound for the West Brom hitman.
But Bednar charged into the box to hammer home a powerful header from Brunt's cross and almost wrap up the win, and promotion at the first time of asking for West Brom.
West Brom were camped in the Bluebirds half as Chris Brunt and Bednar again went close before counter-attacking Cardiff - the Championship's second highest-scorers - completed a battling smash-and-grab win amid the late drama.
Simpson, in only his second game since arriving on loan from Arsenal, was a constant threat and it was no surprise when he latched onto a header from Bednar, before swivelling and lashing home the opener.
Di Matteo has made the most of the Scot's versatility, using him out wide, through the middle, and even in support of lone striker Roman Bednar in the Baggies' last game at Preston - a role Dorrans relished.
Giles Barnes sent a Baggies cross to the far post where Tchoyi passed to Roman Bednar and his effort found its way to Cox, who produced a lovely finish to dink the ball over on-rushing keeper Logan with aplomb.
In the opening minutes of the second-half, Marshall had a taste of things to come as the Scottish international had to be at his best to keep out Bednar as he looked set to score his fourth of the season.
The relentless Albion attacks opened up the Cardiff defence again just before the hour as Bednar's presence created an opening for Dorrans but with a clear sight of goal and from 15 yards out, the Scotsman skewed his left-foot shot wide.
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