Crush the cookies. You can use a blender, food processor, or place them in a bag and crush with a rolling pin.

Add the melted butter and ground cinnamon to the crushed cookies and mix well.

Press the mixture firmly into the bottom of a 8-inch (20 cm) springform pan. Chill in the refrigerator.

In a saucepan, combine the milk, gelatin packets, sugar, custard powder, and another pinch of ground cinnamon. Cook over medium heat, stirring constantly, until the mixture thickens (about 8 minutes).

Arrange some cookies around the edge of the pan if desired, then carefully pour the custard mixture into the pan over the crust, using the back of a spoon to help spread it evenly.

Let the tart cool, then refrigerate until fully set—preferably overnight. Sprinkle with ground cinnamon before serving.

Once chilled and set, slice and serve.