Preheat an outdoor grill to medium-high heat and lightly oil the grate.

Whisk oil, lemon juice, garlic, salt, smoked paprika, brown sugar, chili powder, and black pepper together in a bowl; set aside.

Remove the tough bottom leaves from each cauliflower, taking care not to remove the stem. Place the cauliflower heads onto a cutting board with the stem side facing upwards; cut vertically on both sides of the stem to remove the loose florets. Cut each cauliflower vertically through the stem into 2 even "steaks", 1 to 1 1/2 inches thick; remove any remaining small leaves from each steak. Place cauliflower steaks on a baking sheet.

Brush one side of each steak with some of the oil mixture. Place steaks on the preheated grill, oiled-side down; grill for 5 minutes. Brush more oil mixture on top of steaks and gently flip over; grill until golden brown and slightly charred, 5 to 7 minutes more. Drizzle with any remaining oil and remove to a plate. Sprinkle with cilantro.