Cook elbow macaroni in boiling, salted water for 6 minutes. Drain and set aside.
Preheat oven to 350 degrees. Melt 3 tablespoons of the butter in large saucepan on low heat. Blend in flour; cook and stir 1 minute. Gradually whisk in milk; cook, stirring constantly, until thickened. This will take about 4 to 5 minutes.
Stir in Lawry's Seasoning Salt, mustard and pepper and then add Velveeta and cheddar cheese; stir until melted.
Stir in macaroni, mix well and remove from heat. Pour into a lightly greased 1 ½ quart casserole.
Melt remaining 1 tablespoon butter; toss with bread crumbs. Sprinkle casserole with bread crumb mixture.
Bake for 20 to 25 minutes or until bubbly and heated through.
For a nice browned top, at end of baking time, crack open oven door, turn to broil and cook for an additional 2 or 3 minutes. Watch closely and remove from oven as soon as it is golden brown.