Preheat oven to 350°F. Grease a 9"x9" or 2 quart baking dish with butter or nonstick spray.
Cook macaroni according to the package instructions until al dente. Drain and set aside.
In a large skillet, melt butter over medium heat. Add flour and whisk constantly for about 1 minute.
Gradually pour in the milk while whisking to prevent lumps from forming. Cook until the mixture thickens, about 5 minutes.
Reduce the heat to low. Add the shredded cheddar cheese and pimento cheese to the saucepan, stirring until the cheeses are melted and the sauce is smooth.
Season the cheese sauce with salt and pepper to taste.
Add the cooked macaroni to the cheese sauce, stirring until the pasta is evenly coated.
Pour the mac and cheese mixture into the prepared baking dish, spreading it out evenly. If desired, sprinkle a mixture of 1 cup of crushed round butter crackers and 1/4 cup butter evenly over the top.
Bake 20-25 minutes, or until the cheese is bubbly and the top is golden brown.
Remove from the oven and cool for a couple minutes before serving.