In med pan combine sugar, starch & salt; gradually add milk, stirring constantly. Cook & stir over med heat until very thick & begins to bubble. Remove from heat. Add a few Tbls. Hot mixture to yolks & stir to blend. Slowly pour yolks into pan, stirring constantly. Cook & stir mixture over low heat. Stir in butter, vanilla & cinnamon. Transfer filling to bowl. Place piece of buttered waxed paper directly on filling & cool to room temperature. Refrigerate 1 hour before using.