Melt butter in a saucepan over medium heat. Stir in yeast extract spread, onion powder, and salt until smooth. Gradually whisk in 1 1/2 cups water so no lumps form. Bring to a boil. In a small cup, stir together 1/2 cup cold water and cornstarch. Stir the cornstarch mixture into the saucepan, and continue boiling until thickened. Cool slightly before serving.
