In a small saucepan, add the butter and turn the heat to medium. Once the butter is melted, add the flour. Whisk, until the mixture is smooth and foamy, and smells slightly toasty, cooking for about 1-2 minutes. While whisking, slowly pour in the milk, stirring to combine. Bring the mixture to a simmer, whisking constantly, until the mixture has thickened and there are no clumps. The sauce should be about the consistency of an alfredo sauce.