Preheat the grill and skillet. Get your Weber kettle hot with the cast iron skillet on the grate for at least 10 minutes until it's smoking hot.

Season and portion the beef. Mix chili powder and garlic powder into the ground beef, then divide into 5 loose balls (about 3 oz each). Don't overwork it.

Prep the portobello. Brush the mushroom cap with olive oil and season with a pinch of the same chili powder and garlic powder mix.

Char the corn. Melt butter in the hot skillet, add the corn, and let it sit undisturbed for 2–3 minutes until charred in spots before stirring. Remove and set aside.

Make the sauce. Stir together mayonnaise, lime juice, and chili powder in a small bowl.

Smash the patties. Place beef balls on the hot skillet, press hard with a sturdy spatula for 10–15 seconds to flatten thin, then season the tops with salt and pepper.

Sear the beef and mushroom. Cook the patties 2–3 minutes per side until deeply crusted and cooked through. Sear the portobello alongside for about 3–4 minutes per side until tender.

Toast the buns. Cut buns in half and toast cut-side down in the same skillet in the leftover fat, about 1 minute.

Assemble. Spread lime sauce on both bun halves, add the patty (or sliced portobello), top with charred corn, a sprinkle of parmesan, and cilantro if using.