Preheat the oven to 250°C (482°F).

Pour 120 ml whole milk into a small saucepan with 1 teaspoon vanilla bean paste.

Bring the vanilla milk to a gentle simmer, then remove it from the heat.

Combine 120 g granulated sugar, 80 ml water, and 1 cinnamon stick in a second small saucepan.

Cook the sugar mixture until the sugar dissolves and the syrup reaches a boil.

Whisk 20 g all-purpose flour with 30 ml whole milk in a mixing bowl until smooth.

Slowly whisk the warm vanilla milk into the flour mixture to make a smooth base.

Gradually whisk the hot cinnamon sugar syrup into the custard base.

Strain out the cinnamon stick and let the custard cool until lukewarm.

Whisk 3 egg yolks into the lukewarm custard until fully incorporated.

Roll the puff pastry into a tight log and cut it into 12 equal pieces.

Press one puff pastry piece into each cavity of the muffin pan, forming thin pastry cups.

Fill each pastry cup about three-quarters full with the prepared custard.

Bake the tarts at 250°C (482°F) for 12 to 15 minutes until the pastry is golden and the custard has dark caramelized spots.

Cool the tarts briefly, then serve warm or freshly baked.