In double boiler stirring constantly cook flour and milk, to a soft or med paste, do not worry ! It will be OK if it gets thick to. You can do this step the night before also. I have even forgot to do this and put the recipe "on hold" until I had this done and cooled.
Put paste in bowl, cover, cool in refrigerator, about 1/2 hour or until cooled.
Put paste in mixer bowl, add sugar,butter, vanilla, salt and mix. Scrape and mix some more.
Mix some more until granulated sugar is dissolving. Let rest for 10-15 minutes. I know this sounds funny but the granulated sugar has to dissolve. Add any of your optional flavors now.
Mix for another minute, test for sugar granules between your index and thumb if no sugar crystals your done if not mix a short while longer.
If you use powdered sugar you do not have to mix as long, it will dissolve instantly.
optional, ways I have changed this recipe extract of your choice:
1/2 of a lemon zest and 2 teaspoons lemon juice.
1/4 cup cocoa powder and / or 1 teaspoon instant coffee or espresso powder, more or less to your taste.