In a small pot, whisk together milk, heavy cream, sugar, corn starch, and cinnamon stick. Transfer to a stove and turn on the heat on low, stirring occasionally, until sugar and corn starch is dissolved and mixture is slightly simmering. Turn off the heat and let cool slightly for 5 mins.

In a small bowl, whisk together the egg yolks and vanilla extract. Gradually pour the warm cream mixture into the egg yolks, whisking constantly until fully incorporated. Strain through a fine mesh strainer and set aside.

Preheat your oven to 475F. Arrange the puff pastry egg tart shells on a baking sheet and pour the custard filling into each shell until about ¾ full. Bake for 15-20 minutes until the tops have golden brown spots and the custard is set.