Turn the oven on 200 C
Add butter in the pan, now add flour, stir it vigorously for 1-2 minutes, when the flour starts getting cooked, takes few seconds, add milk in smaller portions, and keep stirring it vigorously.
Now fill the small tarts with sauce and put it in the oven for 10-15 minutes. If prefer more brown edges, can also keep for another 5 minutes more.
I like to keep some remaining sauce to eat with the tarts.
If hosting a party, the sauce could be made a night before and could be kept in the refrigerator.
Could be a great recipe for picnic menu.
Great for kids lunch box