Preheat oven to 425F. Lay a piece of parchment paper on a large baking sheet.
Layer lime slices from first 2 limes in center of parchment paper (save the lime "butts" for later). Place salmon on top of slices (skin side down if using skin-on). Squeeze the juice of the other 2 limes over salmon.
Arrange butter pieces on top of salmon so it gets nicely coated as butter melts. Sprinkle with pepper (and salt and chili powder if desired). Top with lime "butts". Arrange whole cilantro around everything.
Place a second piece of parchment on top. Roll up edges, pressing on rolls as you go to seal. Don't make the pouch too tight on top you want to leave room for steam to circulate.
Place in oven. Bake 20 minutes. Remove from oven. Let rest 5 minutes. Open carefully. Serve with desired sides and additional lime and chopped cilantro if desired. Enjoy!
