• Unbox the puff pastry and leave it on the counter to begin thawing about 45 minutes before baking. Use this interval to prepare the custard filling.
• Make a smooth slurry in a small bowl with 3 tbsp flour and ¼ cup whole milk, then whisk in the 6 yolks and remaining 1 cup milk. Check the slurry for flour lumps before adding the yolks and remaining milk.
• Combine ⅔ cup water, 1⅓ cup sugar, the cinnamon stick, and broad pieces of lemon peel in a small saucepan over medium-high heat, then boil for 1 minute or until the syrup reaches 220°F. The syrup should contain no visible sugar grains when you take it off the heat and remove the cinnamon stick and peel.
• Ladle a thin, steady stream of the milk-and-egg mixture into the hot syrup while whisking continuously, stopping once ⅓ to ½ of the milk mixture has been incorporated. Keep the addition gradual so the mixture blends smoothly rather than forming scrambled egg pieces.
• Whisk the warmed syrup mixture into the milk-and-egg mixture still in the bowl. Continue whisking throughout the addition to keep the custard uniform.
• Pass the custard through a mesh strainer into a clean bowl, then whisk in 1 tsp vanilla and ¼ tsp cinnamon and set it aside. The strained filling should look smooth, without egg fragments or larger aromatic pieces.
• Heat the oven to 475°F and, if desired, wipe the molds lightly with butter rather than cooking spray. The butter coating is optional and should be light.
• Roll the puff pastry to approximately ¼ inch thick, then form a tight log starting along its longest edge. The cut end should show closely packed pastry spirals.
• Slice the log crosswise into discs approximately ½ inch thick and set a disc in each mold. Each round should sit with its spiral facing upward, ready for pressing.
• Dampen both thumbs with water and press the center of each disc outward, then work the pastry up the mold walls. Aim for a thin shell that reaches the top without leaving a thick mound at the bottom.
• Pour custard into each shell until it is ⅔ full, and set individual pastry molds on a cookie sheet before placing them in the oven. Leave the remaining headroom empty to allow for bubbling without overflow.
• Bake for 15–17 minutes, using the cupcake-pan adjustments below when applicable. Look for dark spots across the custard rather than an entirely blackened surface.
• Let the tarts cool before gently releasing them from their molds or cupcake pan. The shells should hold their shape as you lift them out.
• Finish with a light sprinkle of cinnamon and powdered sugar if desired. Keep the dusting light so the blistered tops remain visible.

Special Instructions for Using Cupcake Pans

• Allow an additional 2–3 minutes of baking when using a standard cupcake pan, if needed. The centers should feel firm rather than jiggly.
• Shorten the baking time by 2–3 minutes when using a mini cupcake pan. Check for firm centers in these smaller tarts as well.