In a medium saucepan warm the milk until just before boiling- do not boil the milk. While it is heating up, in a separate bowl mix together the sugar and salt and flour. Add egg yolks. Mix well. Slowly stir in the warm milk mixing as you pour. Return everything back to the saucepan. Heat while stirring until large bubbles form on the surface. Whisk for a few more seconds and then remove from heat. Stir in flavor. Strain mixture and cool completely.