How to Make It
Preheat broiler on HIGH with oven rack 5 inches from heat. Toss peppers with oil and 1⁄2 teaspoon of the salt; arrange in a single layer on an aluminum foil-lined rimmed baking sheet. Broil until tops are lightly charred, 3 to 5 minutes. Let cool to room temperature, 30 minutes. Reduce oven temperature to 425°F.
Flip over peppers so charred side is on bottom; cut and remove top third of each pepper (from stem end to tip). Discard cut-off part, or reserve for another use. Scoop out membranes and seeds, and discard.
Whisk together pepper Jack cheese and milk in the top of a double boiler over simmering water over medium. Whisk until smooth, 5 minutes. Remove from heat; stir in cream cheese, jalapeños, and remaining 1⁄4 teaspoon salt. Spoon a heaping teaspoon of cheese mixture into each pepper. Place on a baking sheet. Bake until tops are golden, 5 minutes.
Meanwhile, melt butter in a skillet over medium. Add panko and pepper; cook, stirring, until golden brown, 3 more minutes. Sprinkle peppers with panko mixture and cilantro.
To Make Ahead:
Place filled peppers in a baking dish; cover and chill 1 day. Bake, covered, at 425˚F for 5 minutes, and then uncover and bake 3 more minutes. Top with panko mixture and cilantro.