Preheat oven to 300˚F and line a sheet pan with parchment paper.
Add milk, sugar, cinnamon stick, vanilla and salt to a small saucepan over medium-low heat and stir. Don't bring to a boil, but heat until the sugar is dissolved. Remove from heat, pour into a shallow bowl and let cool for 10 mins.
Slice croissants into ⅓ to ½-inch pieces (lengthwise) using a serrated knife. It's important that the pieces have even thickness, otherwise they will cook at different rates. You'll likely have 3-4 pieces per croissant.
Dip each croissant slice in the syrup briefly, then let excess moisture drip off. You don't want the croissant to be mushy, just moist. If you soak for too long, use your hand to gently squeeze excess moisture out of the croissant.
Place on sheet pan, spaced at least 1 inch apart. Top with another sheet of parchment paper, then top with another sheet pan to compress the croissants.
Place in preheated oven and bake for about 40-50 minutes. Start checking at 40 minutes – some of the smaller pieces may be done first. Remove them and continue cooking the rest. The croissants should be a deep golden brown when they're finished.
After removing from the oven, sprinkle with cinnamon sugar, then break up into smaller pieces once cooled. Enjoy!
