Prepare the base:
Crush the Marie biscuits finely (use a food processor or place them in a zip bag and crush with a rolling pin).
Mix the crushed biscuits with the melted butter until well combined.
Press the mixture firmly into the base of a square or rectangular dish (about 20×20 cm).
Place in the fridge to set while you make the filling.
Make the filling:
In a small bowl, mix the custard powder with a few tablespoons of milk to form a smooth paste.
In a saucepan, heat the remaining milk and sugar over medium heat until warm (do not boil).
Gradually whisk in the custard paste. Stir continuously until the mixture thickens and coats the back of a spoon.
Remove from the heat and stir in the vanilla and a pinch of cinnamon.
Assemble:
Pour the warm custard mixture over the chilled biscuit base.
Smooth the top with a spatula.
Sprinkle a little extra cinnamon on top, if desired.
Chill & serve:
Refrigerate for at least 3–4 hours (or overnight) until fully set.
Cut into squares and enjoy your creamy, nostalgic milk tart treat!