Beechers Mac and Cheese Sauce

This is from http://www.marthastewart.com/319315/beechers-flagship-cheese-sauce.  Beechers is this fantastic cheese shop in Pike Place Market.  You can watch the big vats of milk being processed into cheese--it's amazing.  J and I took a food tour, and we stopped there for some mac and cheese.  Best. Ever.  And here's the actual recipe: 

Ingredients

• 4 tablespoons (1/2 stick) unsalted butter
• 1/3 cup all-purpose flour
• 3 cups milk
• 14 ounces cheddar cheese, preferably Flagship, grated (about 3 1/2 cups)
• 2 ounces Jack cheese, preferably Just Jack, grated (about 1/2 cup)
• 1/2 teaspoon coarse salt
• 1/4 to 1/2 teaspoon chipotle chile powder
• 1/8 teaspoon garlic powder

Directions

1. Melt butter in a heavy-bottomed saucepan over medium heat; whisk in flour. Continue whisking and cooking for 2 minutes.
2. Slowly add milk, whisking constantly. Cook, stirring frequently, until sauce has thickened, about 10 minutes. Remove from heat. Add cheeses, salt, chile powder, and garlic powder. Stir until cheese has melted and all ingredients are well incorporated, about 3 minutes. Use immediately or refrigerate up to 3 days.
3. To reheat sauce, place in a saucepan over low heat, stirring frequently, until heated through.