How to Make It
In a large bowl, whisk the fish sauce, shallots, soy sauce, garlic, miso, five-spice powder and chile. Add the chicken and turn to coat. Cover with plastic wrap and refrigerate for 2 to 4 hours.
Meanwhile, in a bowl, whisk the sugar with the vinegar and salt until dissolved. Stir in the carrot and let stand at room temperature until lightly pickled, about 30 minutes.
Light a grill and oil the grate, or preheat a grill pan. Remove the chicken from the marinade and grill over moderate heat, turning occasionally, until lightly charred and cooked through, 14 to 16 minutes. Transfer the chicken to a plate and let rest for 5 minutes.
Preheat the oven to 400°. Using your fingers, scoop out some of the bread from the insides of the rolls. Toast the rolls in the oven for 6 minutes, until golden. Let cool slightly. Thinly slice the chicken. Divide 1 tablespoon of mayonnaise between the top and bottom of each roll and arrange the cucumber slices and chicken on the bottoms. Top with the cilantro sprigs, jalapeños and pickled carrot. Close the sandwiches and serve.