In a medium saucepan, melt the butter over medium-low heat. Add the flour and whisk together to form a paste. Cook for 2 minutes while whisking continuously.
Add ¼ cup of the milk and whisk until smooth. Repeat until you’ve added 1 cup of milk. Add remaining milk and stir to combine. Increase heat to medium-high and bring to a boil while whisking constantly.
Let boil one minute and take off the heat.
Add cheese, salt, and Dijon and cayenne pepper, if using. Stir until cheese is melted.
