Prepare the Filling:

In a saucepan, combine the milk and the cinnamon stick. Heat over medium heat until it starts to simmer. Remove from heat and allow the cinnamon to infuse for about 10 minutes. Then, remove the cinnamon stick.
In a separate bowl, whisk together the sugar, flour, cornstarch, eggs, and vanilla extract until smooth.
Slowly pour the warm milk into the egg mixture while continuously whisking to prevent curdling.

Assemble the Tart:

Preheat your oven to 350°F (175°C).
Place the pre-made pie crust or tart shell into a 9-inch (23 cm) tart pan.
Pour the milk mixture into the prepared pastry shell.

Bake:

Place the tart in the preheated oven and bake for 35-40 minutes or until the filling is set and the top is lightly browned.
Remove the tart from the oven and let it cool to room temperature.

Serve:

Once the melktert has cooled, refrigerate it for a few hours or until it's nicely chilled.
Before serving, dust the top of the tart with ground cinnamon.
Slice and serve your delicious homemade melktert.