Remove the steaks from the refrigerator and let rest at room temperature for 30 minutes.
In a small bowl, combine the softened butter, cilantro, garlic, lime zest, salt, pepper, and cayenne, if using. Mash with a fork to combine and mix well. Transfer the butter to a piece of plastic wrap and roll into a little log. Refrigerate for 30 minutes.
Meanwhile, in a small bowl, combine the Tex-Mex seasoning, brown sugar, salt, and ground black pepper.
Brush the steaks all over with the olive oil, then sprinkle on both sides with the spice mixture.
Place in a single layer in an air fryer basket and cook at 400°F for 7 minutes. Flip the steaks and cook for an additional 3 to 7 minutes, or until the temperature of the steak is at around 135°F, for medium-rare.
Cut slices from the butter and set over top of the hot steak. Let rest for 5 minutes, then slice and serve.