In a medium stockpot over high heat, combine the shrimp shells and tail segments, blue crabs, water, and bouillon powder and bring to a boil. Turn the heat to a gentle simmer, cover, and let simmer for 30 minutes.
While the broth simmers, in a large skillet over medium heat, melt the 1 cup butter. Then turn the heat to medium-low and add the yellow onion, bell pepper, celery, garlic, and green onions and cook, stirring frequently, until the vegetables are completely softened and have broken down, about 25 minutes.
Meanwhile, when the broth is ready, remove from the heat, taste for seasoning, and adjust with more bouillon powder if needed. Strain through a fine-mesh sieve, discarding the solids. Set the broth aside.
Back in the large skillet, push the vegetables to the side of the pan. Add the flour, Cajun seasoning, cayenne, and black pepper to the empty side of the pan and cook, stirring regularly, until the flour is nice and toasted, about 4 minutes. Combine the vegetables with the flour mixture and stir to mix thoroughly. Add 5 cups of the broth, a cup or so at a time, stirring constantly to build a gravy. When all the broth is incorporated, cover, turn the heat to low, and simmer the gravy for about 15 minutes. The mixture should have the consistency of a medium-thick gravy.
When the gravy is almost ready, in a medium sauté pan or skillet over medium-high heat, melt the remaining 3 tablespoons butter. Add the shrimp and crawfish meat and cook, stirring frequently, just until they take on color, 3 to 4 minutes.
Using a slotted spoon, transfer the shrimp and crawfish to the gravy and stir to combine. Allow to simmer until just warmed through, 2 to 3 minutes. (I prefer a medium-to-thick gravy for my étouffée, but if it is coming out thicker than you like it, add a little more of the broth.) Taste for seasoning and adjust with bouillon powder if needed.
Serve the étouffée immediately with rice and garnished with the parsley, if desired.
Leftover étouffée will keep in an airtight container in the refrigerator for up to 3 days.
