Preheat the oven to 250°C (482°F). Grease a 12-cup muffin tin.
Roll out the puff pastry and cut out 12 circles to fit the muffin cups. Press the pastry circles into the cups and chill while preparing the custard.
In a small saucepan, whisk together the milk and flour until smooth. Cook over medium heat, stirring constantly, until the mixture thickens, about 5 minutes. Remove from heat and set aside.
In another saucepan, combine the sugar and water and bring to a boil. Cook until the sugar dissolves and the syrup reaches a temperature of 220°F (104°C) on a candy thermometer.
Slowly whisk the syrup into the milk mixture. Stir in the egg yolks and vanilla extract.
Pour the custard into the pastry cups, filling them about three-quarters full.
Bake in the preheated oven for 10-12 minutes, or until the pastry is golden and the custard is set.
Sprinkle with ground cinnamon before serving.
Tips
For an extra caramelized top, place the tarts under the broiler for a minute or two after baking.
