In a bowl, dissolve the cornstarch in the evaporated milk. Set aside.
In a small saucepan, soften the onion, garlic, and jalapeño in the oil. Add the tomato paste and cook for about 1 minute. Add the evaporated milk and bring to a boil, stirring with a whisk. Simmer gently for about 2 minutes. Add the cheese and stir until melted. Season with salt and pepper. Set aside.
Shape four patties with the ground beef.
In a large skillet, brown the patties in the oil for about 3 minutes per side or until the meat is thoroughly cooked. Season with salt and pepper. Toast the buns.
Generously spread cheese sauce inside the buns. Place the meat on the bottom bun halves. Garnish with bacon, shallots, and pickles and close with the top bun halves.
Serve the remaining cheese sauce as a dip with corn chips.