Prepare the ingredients per the instructions above.
Preheat the oven to 400°F (204°C).
Make the guacamole: Mash the avocado in a bowl with a fork until it is smooth. Mix the onion, garlic, lime juice, and cilantro into the bowl. Season with salt and pepper to taste. Refrigerate until ready to serve.
Make the fish: Line a large baking sheet with parchment paper and place the fish on it. In a small dish, combine seasonings: paprika, garlic powder, salt, and black pepper. Evenly sprinkle seasoning mix over both sides of the fish.
Lightly drizzle fish with olive oil and dot each piece with butter. Bake for about 15 min to an internal temperature of 125°F (51°C) using a thermometer. Then gently cut it into pieces.
Combine all sauce ingredients in a medium bowl and whisk until well blended.
In a blender, combine ice cream, milk, and vanilla extract. Blend everything on low speed. If you want your milkshake to be thicker, add more ice cream. Likewise, if too thick, add more milk.
To serve the tacos, quickly warm the taco shells on both sides in a large skillet over medium/high heat.
To assemble: Layer the shells with lettuce, fish, and toppings, and drizzle with sauce. Serve with fresh lime wedges, guacamole with chips, and milkshake.