Line a baking sheet with parchment or foil. In a small dish, combine seasonings (cumin, cayenne, salt & pepper) and evenly sprinkle seasoning mix over both sides of tilapia.
Lightly drizzle fish with olive oil and lay a pat of butter on top of each fillet. Bake at 375 for 20-25 min. To brown edges, broil for 3-5 additional minutes, if desired.
While fish is baking, combine all sauce ingredients in a medium bowl and whisk until well blended.
To serve the tacos, lightly toast the corn tortillas in a large dry skillet or griddle over medium/high heat. Turn to toast both sides, being careful not to burn.
To assemble: add large chunks of fish to taco shell, then toppings, finishing with a generous sprinkle of Cotija cheese and a drizzle of your sauce. Serve with fresh lime wedges to squeeze over tacos.