Place the peppers on a rimmed baking sheet under the broiler and cook for about 8 minutes total, turning them frequently as the side towards the flame gets bubbly and charred. When the peppers are all blistered, place them in a small bowl and place a dishtowel or plate on top to cover. Allow it to steam, and when cool enough to handle, peel off the skin and remove the stem and seeds. Roughly chop the peppers (use gloves or wash your hands very well after with hot, soapy water).
In a small bowl, mix together the chopped jalapeños, jalapeño havarti, cheddar, cream cheese, bacon, garlic powder, salt, and pepper.
Butter one side of each of the pieces of bread. Evenly spread the cheese mixture on 2 slices of the bread, on the un-buttered sides. Cover the sandwiches with the remaining slices of bread, buttered side up. Lightly sprinkle the top with salt.
Heat a large skillet with a lid over medium-high heat. Place the sandwiches in the pan, cover, and cook until the bottom starts to turn golden brown, about 3 minutes. Uncover and carefully turn the sandwiches over with a spatula, then press them down a bit with the spatula and cook uncovered until the bottom is golden brown and crisp, about 2 minutes. Flip one more time, and cook until the bottom crust is fully golden brown and crispy, another minute or two.
Transfer the sandwiches to the cutting board, let them sit for two minutes for the cheese to firm back up a bit, cut in half, and serve. Pass some salsa in a small bowl for dipping if you like.