Preheat over to 550 degrees.
Lightly grease the muffin tins with the butter.
Add the sugar, water, sliced lemon zest, vanilla to a small saucepan. Bring to a boil without stirring. Remove from heat when the temperature reaches 210 degrees Fahrenheit.
In another cold saucepan combine the milk, sifted flour, and salt and whisk. Cook over medium heat for around 5 minutes, constatnly whisking. May need less depending on pan. When well combined and thickened, remove from heat and allow to cool for 10 mintues
Once the milk mixture has cooled, whisk in the egg yolks until well combined.
Remove the cinnamon stick from the sugar syrup and pour into the milk mixture you just made. Mix until well combined. Strain the mixture in a pourable bowl or cup.
Lay your pastry sheet on a cutting board. Roll it to thin it more. Score into 12 even pieces of puff pasty dough.
Place one square into each greased muffin tin. Use the bowl of water to dip your fingers in to mold into form. The top should be 1/8 inch over the edge of the muffin tin.
Fill each pastry cup 3/4 way with the custard filling.
Place in the 550-degree oven and bake until the pastry turns golden brown and the custard started to caramelize and blister, about 10-12 minutes.
Remove from oven and allow to cool for a short time in the muffin pan, 5-10 minutes.
Remove tarts carefully and if desired dust with cinnamon and powdered sugar. Serve while still warm.