Melt butter in a heavy stockpot over medium heat. Add onion, celery, and garlic paste. Cook, stirring, until celery and onions are tender, about 5 minutes.

Add flour to form a roux and stir with a wooden spoon to keep the mixture from sticking to the pot. Cook until roux has browned slightly.

Add Guinness and whisk vigorously to avoid lumps. Stir in heavy cream, whole milk, water, beef bouillon, hot sauce, Worcestershire sauce, cayenne pepper, and white pepper. Cook until slightly bubbling, about 180 degrees; do not boil.

Remove soup from heat and whisk in Karst and parmesan cheeses, a handful at a time, making sure each handful is fully incorporated before adding another. Immediately add soup to a blender and blend until smooth, or use an immersion blender. Add chives and red pepper flakes and season to taste with salt.