This time, however, the honours went to the Scarlets as the prodigious outside-half bolstered his season's tally to 36 points from two games.
A Bradley Thyer try and a conversion and penalty from Rhys Patchell put the Blues 10-9 to the good at half time. However, a second-half penalty from replacement Gavin Thomas was not enough to earn the visitors victory as Williams, who kicked three first-half penalties, added two more in the final quarter to secure a two-point success.
Scarlets team manager Rob Evans said; "It was a terrific game played at a really decent pace. Both sides deserve credit for that, but in the end, it came down to the final kick of the game. Obviously we are delighted with the result and can now look forward to next week's game against the Dragons.
"I thought we had lost it when Jordan pushed a penalty wide of the uprights in the penultimate minute of the game. However, we got one final chance and as he has done on so many occasions, Jordan made no mistake."
Not such good fortune for the Scarlets under 16s who were beaten, for the second time this season, by Blues North at Abercwmboi.
A Jevon Barnes try and three penalties from Jack Maynard were not enough to prevent the visitors from falling to a 32-14 defeat. Matthew Jeczalik (2), Garin Smith, captain Brad Llewellyn and Jonathan Barnett all crossed for tries for the North with Jarrod Evans and Jordan Jones each converting one. Evans added a penalty to round off a decent night for the hosts.
Blues North team manager Nigel Way said; "I'm particularly pleased because we made eight or nine changes in a bid to ensure every member of the squad got some game time.
"I think the performance tonight will keep our players on their toes. It was a great performance and that's three in three for us now."
Meanwhile Blues South under 16s went down 45-19 to the Ospreys at the Llandarcy Academy.
Joseph Bishop, Olly Hitchins and Gus Rees Macpherson helped themselves to tries while Geraint Llewellyn weighed in with two conversions for good measure for Blues South.
It wasn't enough to see the Blues home, though, as the Ospreys ran in six tries through leading scorer Dafydd Howells (2), Karl Leyshon, Daniel Brooks, Liam Popham and Jack Anderson. Outside-half Josh White converted all six and kicked a penalty to finish with 15 points.