Put the porter in a sauce pan and reduce over medium to medium-low heat by half. Don't let it boil rapidly, as this could make the porter taste bitter. Set aside.
Heat a sauce pan over medium heat. Add the butter.
When butter is melted and bubbling, whisk in the flour, mustard powder, grain mustard, salt and pepper. Cook for two minutes.
Add the cream and half and half (if you want a little less fat, by all means use 2 cups of whole milk. Just know that your final sauce won't be quite as thick).
Bring to a boil, whisking well. Boil gently for ten minutes. Add the porter and whisk.
Off the heat, stir in the Taleggio and Parmesan until melted and smooth. Taste and adjust seasonings as necessary.
Serve warm as a dipping sauce for the pretzels. This sauce will keep in the refrigerator for three-four days.
You could certainly substitute a nice ale for the porter and use cheddar cheese instead of Taleggio and Parmesan.
I know you'll love the pretzels, and I encourage you to try this sauce. It might not be exactly what you're used to, but it is excellent. Enjoy!