Roast the poblano peppers under a broiler, over a gas flame, or on an outdoor grill (the latter is my personal favorite way to do this). Turn the peppers occasionally until the skins are blackened all over. Place the roasted peppers in a large bowl and cover tightly (a fitted lid or plastic wrap works well). Set aside for at least 20 minutes, and up to several hours.
Heat a medium saucepan over medium and add 1/2 tbsp of the butter. Add the shrimp shells and tails (DO NOT add the shrimp itself), garlic, and a pinch of the salt. Saute gently until the shells are crisp and bright reddish-pink and the garlic is fragrant; about 3 minutes. Add approximately 1.5 cups of water to the pan, then bring to a boil. Reduce the heat and allow the broth to simmer for 20-25 minutes. Strain the mixture and set the broth aside.
Combine the Mexican crema and buttermilk on the stovetop in a medium-sized pot. Heat gently, but do NOT bring to a boil/simmer (the cream will curdle and split). Add the nutmeg, and bay leaves, then add 1 cup of the shrimp stock and the remaining salt. Adjust the salt to taste. Once the mixture is heated-through (again, not simmering), keep it warm on the lowest stovetop setting you have.
While the sauce is heating (and while the shrimp are cooking if you're particularly good at multi-tasking), heat a heavy, dry cast iron pan or comal over medium heat. Add the tortillas and toast them for about 30 seconds to 1 minute per side. They should brown or blister a little bit, and become pliable. You can do one tortilla at a time, but if you fit more of them into the pan without overlapping them, feel free to do several at once.
Melt the remaining butter (1.5 tbsp) in a frying pan over medium-high heat. Add the diced tomatillo to the pan and saute for 30 seconds. Add the spot prawns and cook evenly until just done - about 3-4 minutes. Make sure to turn the shrimp to cook both sides evenly, and remember that you want the center to be just past that translucent/raw stage.
Peel, seed, and slice the poblano peppers into long strips. Dunk a corn tortilla in the buttermilk sauce, then lay it flat on a plate. Fill with a few prawns, some tomatillo, and a few poblano strips. Spoon a tablespoon of sauce over the mixture and fold the tortilla over (because spot prawns are fairly large, you probably don't want to roll these enchiladas - a simple fold-over will do). Spoon some more sauce over the closed enchilada. Repeat with the remaining tortillas and fillings, and serve immediately.