In a food processor blend the tennis or digestive biscuits with cinnamon and melted butter until combined and crumbled.
Transfer to an 11-inch tart pan with a removable bottom and spread over the bottom and up the sides, firmly pressing down. Place the pan in the fridge to harden while you make the filling.
For the filling:
With an electric mixer or a food processor, mix together the flour, cornstarch, salt, ½ cup milk, sugar, eggs, and vanilla until well blended.
In a large saucepan, add 4 cups of milk and butter and bring to a boil over medium heat. Be careful not to burn the milk.
Lower the heat to low and slowly, in a thin stream, add the flour mixture to the hot milk, whisking constantly. Keep whisking until thickened and completely cooked, about 15 minutes, or when the filling pulls away from the sides of the saucepan.
Pour in the prepared tart pan. Sprinkle with cinnamon sugar and let cool. When cool, refrigerate until firm enough to slice, at least 1 hour before serving.
You can pack the biscuits in a single layer on the bottom of the tart pan, instead of crushing them into crumbs.
Make the crust ahead of time; just simply leave it in the fridge in the tart pan.
Adding a cinnamon stick to the saucepan with milk will infuse the filling with a hint of cinnamon. Remove the stick before pouring the filling over the crust.
Stove-top custard is ready when it's thick enough to coat a whisk or a spoon well, or if you're able to run a spoon through it and the line holds its shape.
Decorate your tart by dusting ground cinnamon over a stencil, paper cut-out, or doily gently placed on top of the tart. Lift carefully.
You can mix up the flavor a bit by using nutmeg and cinnamon instead of just cinnamon.
See post details for biscuits substitutions and a gluten-free version.
Please keep in mind that nutritional information is a rough estimate and can vary significantly based on products used.