Lyon held a 16-3 advantage at the break before Castres staged a fightback.
Thomas Combezou's two tries at the start of the second half brought Castres level and Kockott kept his cool at the end to boot over the winning points with a 40-metre penalty.
Of those who have played two games, Stade Francais is the only other team to have won both matches as it secured a 20-8 home success over Bordeaux-Begles.
It too was behind at the break as its opponents led 8-6 before Waisea Nayacalevu and Kylan Hamdaoui crossed in the second period.
Grenoble has lost both of their first two fixtures as Toulouse secured a 23-20 success, with Thomas Ramos kicking three penalties and converting each of his team's two tries.
Montpellier ran in four tries in a 36-14 triumph over La Rochelle while Colin Slade kicked four penalties in Pau's 20-10 success over Toulon.