Put the buttermilk in a large mixing bowl (or covered container) with a small handful of salt. Add the chicken pieces and make sure that they’re fully submerged. Cover with plastic (or the lid) and refrigerate for at least 4 hours, but no more than 10.
Sometime while the chicken is brining, make the Lemongrass-Fish-Sauce Caramel. Cool and refrigerate until you’re ready to use it.
Remove the chicken from the fridge. Put 2 to 3 inches of oil in a large, heavy-bottomed Dutch oven or skillet over medium-high heat. You’ll start frying at 325 °F, so keep an eye on the temperature; I like to use a candy thermometer clipped to the side of the pot. Heat the oven to 200°F and fit a rimmed baking sheet with a wire rack.
While the oil heats, put the flour in a large mixing bowl; add a big pinch of salt and stir to combine. Add about 1/4 cup of the buttermilk brine to the flour, and stir it around so that little, pea-sized clumps of flour form. These clumps, when packed onto the chicken, are what will make the crust crunchy and craggy. One piece at a time, transfer the chicken to the flour mixture; press and pack the flour all over so that the chicken is thoroughly coated. Shake off any excess flour and transfer the chicken to a sheet pan.
When the oil reaches 325°F, it’s time to fry. Cook the chicken in two batches; thighs and legs in one batch, and breasts and wings in the other. As soon as you add the chicken, the oil temperature will drop, and that’s fine; it will climb back to 325°F as the chicken cooks. For both batches, cook, checking and turning the pieces occasionally, until they’re crisp and golden all over and no longer pink at the bone. Thighs and legs will take around 15 minutes; breasts and wings will take around 12 minutes. Transfer the cooked chicken to the prepared pan and keep it warm in the oven while you cook the remaining pieces.
Put the Lemongrass-Fish-Sauce Caramel in a small saucepan over medium heat and cook just until it begins to bubble (you just want to warm it up so it isn’t cold). To coat the chicken in the caramel, either dip it directly in the saucepan using tongs, or your fingers if you can stand the heat, or “paint” the caramel onto each piece of chicken with a brush. Transfer each piece of caramel-coated chicken to a platter, sprinkle with scallions if you’re using them, and serve immediately.