Bartlett-Robinson also uncorked a pinpoint cross-kick for winger Lennix Tovo to leap spectacularly right above his opposite Peceli Malanicagi's shoulders to catch and score in the corner, while hooker Bryn Gordon was set at the back of a penalty lineout drive to become his team's top try-scorer this season with four.
Cool goal-kicking halfback Jack Tatu-Robertson added one conversion and an excellent long-range penalty in between the tries to have the visitors eyeing up just their second-ever Bruce Steel win in Whanganui.
But poke the bear and it gets angry, as Whanganui secured the restart and centre Kameli Kuruyabaki immediately began to impose himself with powerful runs, breaking the defensive line.
This led to penalties for Whanganui to score back-to-back tries through No 8 Semi Vodosese and then winger Alekesio Vakarorogo after they turned down a kickable three-pointer right on the break.
Even at 15-14, first five Dane Whale in consultation with skipper Campbell Hart was keener on going to the sideline for penalties instead of the goalposts, and the forward pack rewarded them as Vodosese got his double and new Canterbury import Mathew Taula-Fili was the man at the back for two drive-over tries from a lineout and scrum.
Whale and Horrocks took control of the territory, with Whale securing Whanganui's first 50m-22m lineout under the new rules, and then scoring the last try under the posts following a long build-up.
Malanicagi backed up Kuruyabaki on their breakouts, as did reserve Timoci Seruwalu when he came on to play his old team – Whanganui hunting tries rather than extra kicks at that point with Ethan Robinson replaced after another good day with the boot.
Teenager Ezra Malo ended up playing nearly three-quarters of the game at fullback, as in another blow, former Highlander Craig Clare's hip injury was further aggravated.
"He done it last week against King Country when we scored that try with Peceli out in the wide corner, and he dropped down," said coach Jason Caskey.
"He said he just got contact and the whole leg just went numb. I don't think it's probably looking good for the short term, I imagine it's going to be a couple of weeks.
"So we're sweating on [Te Rangatira Waitokia returning]. But I doubt we're a chance of getting him this week, probably another week yet.
"We just [need] to take stock during the week, and it might be Ezra's big opportunity. You get the chance, you take it."
It was not quite a "game of two halves" - more 30 minutes of bad rugby being offset by 50 minutes of very strong football.
"We were just a little bit loose in the carry; we probably got stripped or lost it 5-6 times in that first half in contact," said Caskey.
"Gabe played well, carried well, but he must have lost a couple. Lindsay I think [as well].
"We had a little bit too much daylight and they were getting an opportunity over the ball.
"Once we got into it and upped our intensity in the last 5-10 minutes of the first half, we looked pretty good.
"I was confident that so long as we kicked on like that in the second half, we'd be fine."
Leading that revival had been Kuruyabaki, as his ability to break tackles and keep his speed up got Whanganui on the front foot, while a belting tackle and turnover on Horowhenua-Kapiti fullback Himiona Henare was a second-half highlight.
"I thought he was really good today, best on the day. He carried well, run hard and he made some really good defensive reads and got up, drove over the ball," said Caskey.
However, amongst the pack, the skipper Hart is still looking for the eight to get early momentum, rather than chase the game.
"That was pretty disappointing really, we've talked about it a lot – getting our cleanouts right and getting our carry right.
"Both of those tries in the first half came off those sort of errors, so soft by us.
"That's always the thing with rugby, you've got to go forward before you can go wide - maybe sometimes we forget that."
Still, the Sir Colin Meads and now Bruce Steel memorial trophies have been secured for 2021, so now it is onto the big trophies in either the Lochore or Meads cups.
"It was probably one of the first goals we put on the list – team goals – so it's good to get that one done."
Whanganui 40 (Semi Vodosese 2, Mathew Taula-Fili 2, Alekesio Vakarorogo, Dane Whale tries; Ethan Robinson 3 con, Whale 2 con) bt Horowhenua-Kapiti 15 (Lennix Tovo, Bryn Gordon tries; Jack Tatu-Robertson pen, con). HT: 15-14 Horowhenua-Kapiti.
