Heat Oven and Prepare Baking Dish
If baking and not serving stovetop mac and cheese, heat oven to 375 degrees F (190 degrees C). Butter, or spray with non-stick cooking spray, a 3-quart casserole dish.
Cook Pasta
Bring a large pot of salted water to the boil, add pasta then follow package directions, but cook 1 minute less than the package calls for. Drain then rinse pasta with cold water.
Prepare Cheese Sauce
Add milk to a large microwave-safe measuring jug and microwave 1 to 3 minutes until warm. Alternately, you can add milk to a large saucepan over medium heat then heat milk until warm.
Melt butter in a large, high-sided pan over medium heat. When the butter begins to bubble, add the flour. Cook, whisking constantly 2 to 3 minutes until butter smells fragrant and nutty -- the color of the butter-flour mixture will be light brown.
While whisking, slowly pour warm milk into butter and flour mixture. Continue to cook, whisking constantly until the sauce bubbles and thickens.