Whereas whisking continually, slowly pour within the 3 cups milk. Enhance the warmth to medium-high and produce to a boil, stirring continually. Scale back the warmth to keep up a simmer and prepare dinner, whisking usually, till the sauce thickens considerably, 5 to 10 minutes. Take away from the warmth. Add 1/2 teaspoon paprika, 1/4 teaspoon dry mustard, the remaining 1/4 teaspoon kosher salt, and 1/4 teaspoon black pepper, and whisk to mix.