Melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook, stirring constantly, for 1 minute until bubbly.

Slowly pour in the warm milk, whisking until smooth. Cook for about 2 minutes until the sauce thickens and coats a spoon. Remove from heat, then stir in the shredded cheeses one handful at a time until completely melted and glossy.

Stir in most of the jalapeños, the brine, and a pinch of salt. Transfer to a serving bowl, top with the remaining jalapeños, and serve immediately with tortilla chips.