Pre-heat the oven to 350. In a medium saucepan, bring the butter over medium-low heat until melted. Add the flour a little at a time, whisking it into the butter until combined. Add the milk a little bit at a time, whisking it into the flour mixture until smooth and well combined. Turn the heat up to medium for a few minutes until the milk is warmed. Add the salt, pepper, mustard powder and 6 ounces of the shredded cheese (reserve 2 oz for topping) and whisk the mixture together. Continue to heat for another 3-5 minutes, whisking periodically, until the cheese is melted into the sauce and sauce has thickened.