The outside of the Brodnowski hospital in Warsaw looks pretty unassuming.
But inside this building, in a purpose-built wing of the hospital, is the most advanced Interventional Neurology Centre in Europe, and second only to a sister facility in California.
Granada Reports have travelled over to Poland with the family and have been given exclusive access to film the operation.
Hallie's family have taken a huge gamble; raising £70,000 for the treatment that's unlicensed and is currently still being trialled.
As she's wheeled down the corridor to the operating theatre, her family give her one last kiss before they watch the doors close. They know they now have an anxious wait - for what could be ten hours - as the surgeons undertake only the eighth such operation on a child with this condition.
As the family sit and wait, Hallie's 9-year-old sister writes the most touching letter
We've been given incredible access to film Hallie's operation.
The procedure is called gene therapy and involves inserting a bit of DNA into Hallie's brain to encourage her body to start making the enzyme that is currently not working.
The first part of the operation is to drill holes into the skull. Then Hallie is wheeled into a scanner where surgeons must pinpoint with microscopic precision where to deliver the new gene inside her brain. That infusion is done incredibly slowly.
This operation is so new that Hallie is only the 8th child to have it here in Poland. It's groundbreaking stuff and not only could this have huge implications for children like Hallie but also all sorts of other Parkinson-like conditions.
We are watching and filming the operation inside the theatre complex from three viewing windows.
The operation takes seven hours - surgeons tell us that this has in fact gone a lot quicker than their previous operations.
Each time they operate they say they're learning more and improving their techniques. It just shows you how experimental this procedure is.
But that said, watching the team work, you would never think this was such high-stakes surgery; the team are calm and collected - taking regular breaks and tucking into Polish sausage and pickle in the nearby common room before scrubbing up to re-enter the theatre.
The professor running this theatre - who is based in America where the trial of this procedure is currently ongoing - is incredibly modest. He tells me he has wanted to offer this surgery for 20 years but they weren't ready until now.
They've only been undertaking this surgery here for patients with AADC deficiency since January. He says he hopes this gene therapy could potentially be used to help patients with similar Parkinson-like conditions in the future - he's also operated on people with brain tumours, with exciting results.
As Hallie is woken up from the operation she's wheeled out of theatre and back down the corridor to her private room.
She suffers another seizure which doctors say is to be expected. But they are hopeful that this operation will eventually stop these seizures for good - and potentially pave the way for her to learn how to walk and talk.
Hallie's family are told that her condition is likely to get worse before they start seeing the improvements. She has weeks and months of rehabilitation ahead of her.
But several days after the operation, Hallie is doing incredibly well.
She has had her stitches out and been discharged from the hospital and so far, her seizures appear to have stopped.
That incredible smile that inspired so many Granada Reports viewers to donate money for her operation is on full show.
This little six-year-old has probably no idea how significant the surgery that she's just undergone; but her family are all too aware.
Their fundraising campaign was called 'Hope for Hallie', and that they say is exactly what she now has, HOPE.