It took Borough only three minutes after the restart to go ahead as Cicely Holmes released Brown one-on-one and she finished well beyond the advancing Eustace to give her side the lead.
Ten minutes further on and it was deja vu: a ball in behind and Brown burst clear to sweep home a superb finish from close range to extend Borough’s suddenly commanding advantage.
Eustace proved an impressive figure between the posts for Cuckfield as she held on to Tucker’s fizzing strike from distance before she came to Cosmos rescue again, this time to keep out Harrod’s inswinging, wind assisted free-kick.
But there was nothing the keeper could do in the 66th minute as Boro - and their skipper - put the result beyond any lingering doubt. Harrod collected the ball in space wide on the right and curled home a superb effort beyond the reach of the helpless Eustace for the third goal in 23 minutes.
In a compelling and increasingly crazy contest, there was barely time to draw breath as Ellie Ryder pulled a goal back for Cuckfield to the delight of their vociferous faithful, but just when it seemed as though there might be a grandstand finish, up stepped Harrod with the best goal of the day.
There seemed little on when she looked up 35 yards out, but the ball - ably assisted by the swirling Culver Road winds - kept on going and dropped over Eustace into the net for a goal of the season contender.
In what must surely be a first, the day’s two county finals both saw goals direct from corner-kicks when Harrod completed her hat-trick on 73 minutes, set up again by the freak impact of Mother Nature.
The tie had gone mad as the game did its best to keep the shivering Culver Road crowd warm but still the goalfest was not over. Next it was Brown’s turn to notch her hat-trick with a stunning strike from distance to make it 6-1, but to their credit Cosmos never gave up and the mid Sussex side grabbed a consolation in the final minute through Annabel Le Neve - Foster’s delightful lobbed finish.
And so it was Borough secured not only the trophy but a unique piece of history to boot as Daniel Fogg’s side became the competition’s first ever champions on a freezing night at Sussex HQ.
The Eastbourne manager, grinning from ear to ear, praised his side’s quality and intensity in the aftermath of victory. He said: “There were some great goals in there, weren’t there, it’s unusual to have two players get hat-tricks in the same game but Poppy and Willow are two very good players, we try and look to those two girls as our focal point and play to their strengths.
“We used the wind to our advantage, kept the intensity up and our qualities came through in the end. The girls have been excited but focused, we told them to play the game and not the occasion.
"I’m very pleased and delighted to win this trophy - we won it at under-12 level but some of the players have never done this before so together they’ve created a legacy.”