Make corn salsa: Heat a dry large cast-iron skillet over moderately high heat until hot, then pan-roast corn, stirring occasionally, until golden brown, 8-10 minutes .
Transfer to a bowl.
Cook white part of scallions in butter with garlic, 1 teaspoon salt, 1/2 teaspoon each cumin and chili powder, and 1/4 teaspoon pepper in skillet over moderate heat, stirring, until scallions are tender, 3-4 minutes .
Remove from heat and stir in corn, tomatoes, and jalapeños.
Grill steak: Combine remaining 1/2 teaspoon salt, 1 teaspoon cumin, 1/2 teaspoon chili powder, and 1/4 teaspoon pepper and sprinkle on both sides of steak.
Grill, turning once, until an instant-read thermometer inserted horizontally into thickest part of meat registers 130°F, 18-20 minutes total for medium-rare.
Transfer steak to a grooved cutting board and let stand 5-10 minutes before slicing.
While steak is standing, reheat corn mixture over moderate heat, stirring occasionally.
Stir in cilantro and scallion greens.
Spoon corn on top of sliced steak and pour over any accumulated juices.