I am having trouble getting my frosting to "come together" during the beating phase.

I use:
1 cup milk
1/4 cup flour
1/4 tsp. salt

Cooked and cooled.

Then I add:
1 cup butter
1 cup sugar
1 tsp. vanilla

This recipe is watery and has lumps. It has a granulated texture and never whips into a whipping cream consistency - no matter how long I beat it. Any suggestions?