Grease a 12-cup muffin tin.

Using the top of a drinking glass, cut out circles into the puff pastry dough or puff pastry sheets.

Place the circles into the bottom cavities of a muffin pan, pushing the dough against the bottom and up the sides of the cup until it reaches least 1/8 inch past the top. Set aside once all of the muffin tins have been filled.

In a medium sized pot, combine sugar, 1/4 cup plus 1 tablespoon water, cinnamon, and lemon zest.

Boil over medium heat, without stirring, until syrup reaches 210 to 215 degrees F.

Remove from heat.

Preheat oven 550 degrees F.

Whisk flour, salt, and cold milk together very thoroughly in a cold pot. Cook over medium heat, whisking constantly, until milk thickens, about 5 minutes. Remove from heat and let cool for at least 10 minutes.

Whisk egg yolks into the cooled milk. Add the sugar syrup and vanilla extract. Mix until combined. Strain custard into a glass measuring cup.

Fill each cup 3/4 of the way with custard.

Bake in the preheated oven until the pastry is browned and bubbly, and the tops start to blister and caramelize, about 12 minutes.

Cool tarts briefly and serve warm.