The South African scored a 30-metre try to keep Leinster in the contest before half-time at the RDS but the French side left with nothing after a Jack McGrath score just past the hour saw Leinster home.
Pedrie Wannennburg thought he had rescued a potentially crucial losing bonus point for the visitors moments before the end but the TMO ruled that the former Ulster back row hadn't quite reached the try line.
Kockott was one of his side's star performers in the Irish capital, four-and-a-half months after he guided unfancied Castres to the Top 14 title.
Castres shocked Heineken Cup winners RC Toulon to win their first domestic crown in 20 years last term, with Kockott a try scorer on that occasion as well. The 27-year-old kicked eight points in that final having been a major influence throughout the season for the Stade Pierre Antione side.
Kockott has been heavily linked with a switch to Toulon when his contract with Castres runs out at the end of the current campaign, but his agent has denied that any deal has already been done.
Talks on his future aren't expected until November, with Castres then facing the Ospreys in a Heineken Cup double header the following month as they look to take their domestic success on to the European stage.