The 19 year-old left United last summer and had been on extended trial with Jong AZ. Now he has earned a two-year contract.
"It makes me proud. I am very happy with the contract and that I have this opportunity," said Baars.
"AZ is a warm club with nice people who want to help you. I also already knew a number of boys on the team. This allowed me to quickly adjust myself here."
AZ youth chief Paul Brandenburg added: “Millen is a fast-paced and creative attacking player who can manage on both sides. He is technically skilled, athletic and calm in front of goal."