Preheat grill to medium-high.
Combine cream cheese, shredded cheese and jalapeño in a medium bowl, stirring and mashing with a fork. Divide the mixture into 4 portions and form each into a 3-inch disk. Form sirloin into four patties about 4 inches wide and 1/2 inch thick. Season with salt.
Grill the burgers until almost done, 3 to 4 minutes per side. Place a cheese disk on each and continue cooking until an instant-read thermometer inserted in the thickest part of a burger registers 160 degrees F and the cheese starts to melt, 1 to 2 minutes more. During the last few minutes, toast buns on the grill.
Top each burger with 1 tablespoon ketchup and serve on the buns.
Tip: These days, more flavored Cheddar cheeses (like jalapeño, chipotle or habanero) are filling up the specialty cheese section. But easy-to-find pepper Jack works great in this recipe too.
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