In a small saucepan, whisk together milk and flour. Cook over medium- low heat, whisking constantly, until thickened, whisk leaves lines on bottom of pan, and an instant-read thermometer registers 150°F (66°C). (Mixture will look like mashed potatoes.) Transfer to a small bowl, and let cool until an instant-read thermometer registers 130°F (54°C) or until room temperature.