Melt butter in a saucepan
Add the flour and whisk for about one minute until it’s smooth, then add salt to taste before pouring milk into the mixture so there won’t be any lumps from adding cold liquid directly onto hot ingredients.
Whisk continuously while heating up slowly - this will help prevent burning!
Add the cheese slowly while constantly stirring until melted and smooth.
Add mustard powder if desired, then taste test before adding any additional salt or pepper because aged goudas tend to be salty already, so you may not need as much seasoning here.
Add pasta water at the last minute for desired consistency.
Pasta water is starchy and will help thicken the cheese sauce. You only need about ¼ cup, but you can add more or less depending on how thick you want your sauce to be. Just make sure to add it gradually while stirring so that it doesn’t become lumpy.
