Thaw frozen puff pastry according to package instructions, roll out until about 1/8 inch thick, and shape into a jelly roll. Slice into 12 equal pieces.

Press each pastry piece into a greased muffin tin, overlapping slightly at the edges. Chill in the refrigerator for about 30 minutes.

Whisk together sugar and flour in a saucepan, then gradually add milk while stirring. Add cinnamon stick and lemon peel. Cook until thickened, about 5 to 7 minutes.

Whisk egg yolks in a bowl, slowly pour the warm custard mixture into yolks while whisking continuously, then strain the mixture through a sieve.

Preheat oven to 500°F (260°C). Pour custard into pastry cups, filling ¾ full. Bake for 12 to 15 minutes until browned.

Let tarts cool in the muffin tin for 5 minutes, then transfer to a wire rack. Serve warm, sprinkled with powdered sugar and cinnamon.