Unroll the puff pastry on a lightly floured surface. Cut and roll into logs, then into 24 pieces. Press each piece into muffin cups to form pastry shells.

In a saucepan, whisk together milk, sugar, and flour. Cook over medium heat until thickened, about 5-7 minutes. Remove from heat.

In a separate bowl, whisk egg yolks, heavy cream, lemon juice, lemon zest, and salt. Temper eggs by gradually whisking in hot milk mixture. Pour egg mixture into saucepan.

Return to low heat and cook, stirring constantly, until custard thickens and coats the back of a spoon, about 5-7 minutes. Do not boil. Cool slightly for 10 minutes.

Preheat oven to 425°F (220°C). Pour warm lemon custard into pastry shells, filling almost to the top.

Bake for 20-25 minutes, until pastry is golden brown and custard is set with dark spots on top.

Cool in muffin tin for a few minutes, then transfer to a wire rack to cool completely. Dust with powdered sugar and cinnamon if desired.