Mix the chili powder, salt, sugar and cayenne in a wide-bottomed bowl or plate. Heat a cast-iron pan over high heat. Once the pan is very hot, working in batches, coat each side of the watermelon wedges with the spice mixture and place flat in the pan until charred, 1 to 2 minutes. Flip and char the other side of each wedge, 1 to 2 minutes more.
Recipe courtesy of Pablo Munoz