In a medium bowl, whisk together olive oil, lime juice, chili powder, ground cumin, garlic powder, and salt until well combined. Add cod fillets to the marinade, cover, and refrigerate for 20-40 minutes.

Prepare the creamy taco sauce by mixing mayonnaise, sour cream, lime juice, sriracha, garlic powder, smoked paprika, and a pinch of salt in a separate bowl. Refrigerate to meld flavors.

Finely shred cabbage, chop red onion and cilantro, and dice jalapeño if using. Warm corn tortillas in a skillet for about 30 seconds per side to make them pliable.

Heat a skillet over medium-high heat and add butter. Once melted, add marinated cod fillets, cooking for 3-5 minutes per side until flaky and golden. Remove and let rest briefly.

Assemble each taco with a warm tortilla, flaky fish, shredded cabbage, red onion, and cilantro. Drizzle with taco sauce and sprinkle with Cotija cheese or jalapeños if desired. Serve immediately with lime wedges.