Melt 4 tablespoons unsalted butter in a medium saucepan over medium-low heat. Add ¼ cup all-purpose flour and whisk continuously until the mixture has a toasty aroma and becomes a deep golden hue, 3 to 5 minutes. (The roux will thin out a bit as it continues to cook.)
Whisking constantly, slowly add 2 cups milk a ½ cup at a time. The mixture will seize up at first. Don’t worry, just keep slowly adding the milk and whisking constantly — it will become smooth and creamy. Add ½ teaspoon kosher salt and ¼ teaspoon black pepper, and whisk to combine.
Continue to whisk and cook until the gravy is thickened to your desired consistency, 3 to 5 minutes. It should fall in a thick stream from a spoon, but not clump. Taste and season with more kosher salt and black pepper as needed.
