The Rams suffered a miserable 5-2 drubbing at the City Ground in the reverse Championship fixture at the end of December, when Earnshaw helped himself to two goals.
And the Wales international piled more misery on the club he left to join Forest with a 79th-minute winner.
Earnshaw, who replaced fellow former Ram Marcus Tudgay after 66 minutes, controlled Paul Anderson's lay-off before driving a shot into the bottom corner from the edge of the area.
The victory, a fourth in succession in all competitions, temporarily lifted Billy Davies' side into the Championship play-off positions.
It was an eighth defeat in 10 matches for Derby, who finished the game with 10 men following Dean Moxey's red card in injury-time.
Ben Davies was given a baptism of fire as he made his Derby debut barely 48 hours after arriving from Notts County.
The midfielder's inclusion was one of three changes made by Rams boss Nigel Clough, who also recalled defensive duo Moxey and Dean Leacock.
After making an impact from the bench in Forest's late victory over Portsmouth last weekend, top scorer Lewis McGugan and fellow forward Nathan Tyson were restored to the starting line-up.
Typical of an East Midlands derby, the game started at a frenetic pace.
Forest thought they should have been awarded a penalty in the eighth minute when Joel Lynch tangled with Leacock as a corner came into the box but referee Graham Salisbury ignored the appeals.
Stephen Bywater got down well when McGugan's 20-yard free-kick zipped off the greasy surface after 16 minutes. The Rams goalkeeper pushed the ball safe and the danger was cleared.
At the other end, Alberto Bueno looked certain to put Derby ahead until Luke Chambers made a superb goal-saving block.
Tyson flashed a left-footed shot just wide before the break.
It was Forest's turn to spurn a golden chance to take the lead early in the second half when McGugan went through one-on-one with Bywater, but a poor finish from the forward prevented him from adding to his 10 goals this season.
Davies was denied a debut goal when Lee Camp tipped his 12-yard shot round a post after 69 minutes.
Ten minutes later, Earnshaw netted only his fourth goal of the season.
Davies was thwarted by Camp again in the closing stages before frustration got the better of Moxey and he lunged in at Earnshaw.
It was the left-back's second yellow card of the game and Derby ended the contest with 10 men.