Preheat oven to 425 degrees F.
Spray a small baking sheet with non stick spray and set aside.
Remove the stem from the jalapeños, then slice in half and remove the seeds and membranes.
Lay the jalapeños on the baking sheet skin side facing up.
Roast in the oven for 15-20 minutes until skin has blackened and peppers are soft.
Remove the peppers from the oven and using tongs, place them in a container with a lid to allow them to steam for several minutes.
Once the peppers have cooled to the touch, remove them from the container and peel the blackened skins off of them. Roughly chop them and set aside.
In a medium size saucepan, pour 1 cup of milk. Add the salt and garlic powder. Stir to combine.
Heat over medium heat, stirring frequently, until the milk on the edges starts to bubble.
Add in the chopped American cheese in batches, stirring while the cheese melts.
Once the American cheese has completely melted, add in the Monterey Jack cheese as needed until the cheese has a nice, thick consistency.
Stir in the chopped jalapeños.
Serve the cheese warm with tortilla chips.