Step 1: Preheat the oven

Preheat your oven to 200°C (400°F). Grease a muffin tin or line it with non-stick spray.

Step 2: Prepare the puff pastry

Roll out the puff pastry into a rectangle. Roll it tightly into a log and slice it into 12 equal pieces. Flatten each piece into a circle and press it into the muffin tin, creating a pastry shell.

Step 3: Make the custard

In a saucepan, combine the milk, caster sugar, and cinnamon stick. Heat gently over medium heat until the sugar dissolves and the milk is infused with cinnamon. Remove the cinnamon stick and set aside.

Step 4: Thicken the custard

In a mixing bowl, whisk the egg yolks, plain flour, and vanilla extract together until smooth. Gradually add the warm milk mixture while whisking continuously. Return the mixture to the saucepan and cook over low heat, stirring constantly, until the custard thickens.

Step 5: Fill the pastry shells

Pour the custard into the prepared pastry shells, filling them almost to the top.

Step 6: Bake

Bake in the preheated oven for 15-20 minutes, or until the custard is set and the tops are lightly caramelized.

Step 7: Cool and serve

Allow the tarts to cool in the tin for a few minutes before transferring them to a wire rack. Dust with icing sugar if desired and serve warm or at room temperature.