Whisk chiles, tamarind, brown sugar, salt, and 1 Tbsp. oil in a shallow baking dish until sugar is dissolved. Add steak and turn to coat. Cover and chill at least 4 hours.
Heat 2 tsp. oil in a large skillet over high heat. Working in 2 batches and adding remaining 2 tsp. oil between batches, cook steak until deeply browned, 2–4 minutes per side for medium-rare.
Transfer steak to a cutting board; let rest at least 5 minutes before slicing.
DO AHEAD-
Steak can be marinated 1 day ahead. Keep chilled.
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