Add all ingredients to a large skillet and simmer, covered, over medium-low heat, stirring occasionally while you make the cheese sauce. Reduce heat to low after 5 minutes; continue simmering until ready to serve. Remove from heat.
Toss the cheeses with the cornstarch in a bowl to combine, then transfer to a medium, heavy-bottomed saucepan. Add in 2 cups of the evaporated milk along with the jalapeño juice or hot sauce (if using). Place the saucepan over low heat. Use a whisk to stir constantly for 5 minutes, or until the cheese sauce has smoothed out and thickened (it will look pretty gnarly at first). If the sauce is too thick, add additional evaporated milk until it reaches the consistency you like (add in small amounts until the sauce looks right to you).
If you have a multi-compartment Nesco-type food warmer, place the meat mixture in one compartment and the cheese sauce in the other at the lowest setting (or use 2 crockpots). You can also serve from the stovetop, with both the meat mixture and cheese sauce on the lowest setting (stir the sauce frequently). Set out bowls, chips, the meat and cheese, toppings, and condiments and let guests serve themselves (I always advise my guests to give the cheese sauce a stir before scooping it up). Enjoy!
*Absolutely DO NOT use pre-shredded cheese for this recipe: these contain additional starch to prevent the shreds from clumping up in the bag. It's impossible to calculate the necessary adjustments for accommodating that extra starch: suffice to say, it will destroy your sauce. Instead, buy good-quality block cheese and grate it yourself with a box grater (you can do this in advance and keep the cheese in a ziploc until "go time").
**Keep any extra evaporated milk in a plastic container in the refrigerator in case the sauce needs to be thinned out (particularly when reheating).