Melt butter in a heavy saucepan over low heat. Add flour; stir until smooth. Cook 1 minute; stir constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly.
Combine egg yolks, lemon juice, sugar, and salt.
Gradually stir a small amount of hot milk mixture into egg mixture; add to remaining hot milk mixture, stirring constantly. Add whipping cream; blend well. Sprinkle with cinnamon.