Preheat oven to 500°F (250°C).
Making the Custard: In a small pan, add the sugar, water, and cinnamon stick. Bring to a simmer and allow to simmer for 3 minutes. Turn off the heat and set it aside to infuse.
In a separate saucepan, combine milk and cream and place over medium heat. Allow the mixture to come to a simmer, then remove from the heat and set aside to cool down.
To a large bowl, add the egg yolks, whole egg, vanilla extract, and cornflour. Whisk until fully combined and no lumps remain.
Temper the egg mixture by introducing the hot milk to it whisking as you go. Return this egg mixture to the saucepan and place over low-medium heat. Whisk continually until mixture thickens, around 3-4 minutes. Remove from the heat.
Whisk in the cooled cinnamon sugar syrup. Discard the cinnamon stick and set the custard aside.
Assembling the Tarts: Lightly flour a work surface and cut puff pastry into roughly 14 equal rounds using a 3 ½ inch cookie cutter.
To a lightly greased muffin tin, push each piece of puff pastry down and up into the sides of the muffin holes until evenly distributed.
Pour custard into each muffin tin so each one is three-quarters full.
Bake for roughly 15 minutes or until golden brown on top. (See notes above about browning the tops????)
Serve warm as is or with a dollop of cream on top. Enjoy!