Preheat oven to 400 degrees Fahrenheit.
Boil milk, lemon rind and cinnamon together over med-high to high heat.
Remove lemon rind.
Add sugar and flour to milk and stir until sugar is dissolved.
Set mixture aside to cool for at least 10 minutes.
While this is cooling, roll one sheet of the puff pastry shell and cut into 12 equal pieces of dough.
Spray cups of muffin pan with cooking spray.
Squish-flatten the 12 pieces of dough on a flour surface as thin as you can get them without holes and fit them into the cups of the muffin pan.
Return mixture to stove and add egg yolks and vanilla, heating over med-high heat until thick.
Once the mixture is like a pudding texture, scoop 2 tablespoons of mixture into each muffin pocket. Don’t fill any cup more than halfway!
Cook at 400 degrees Fahrenheit for 22 minutes.
Repeat steps, cutting second pastry shell, lining the muffin pan, and filling with the remaining custard to make the second dozen.