Sprinkle steak all over with black pepper and 1 ½ teaspoons salt.
Heat a large cast-iron skillet over high; add oil and half of the steak pieces in a single layer.
Cook, undisturbed, until well browned on bottom side, about 1 minute.
Flip steak pieces; cook, turning occasionally, until most sides are well browned and steak is cooked to desired degree of doneness (1 to 2 minutes for medium-rare).
Transfer steak to a large plate.
Repeat cooking process with remaining steak pieces; transfer cooked steak to plate with first batch.
Remove skillet from heat (do not wipe clean); let cool for 1 minute.
Return skillet to heat over medium; add garlic and cumin to steak drippings in skillet.
Cook, stirring constantly, until fragrant, about 30 seconds.
Add tequila, chiles, and adobo sauce.
Cook, stirring often and scraping up browned bits from the bottom of skillet, until liquid is reduced by half, about 30 seconds.
Remove from heat.
Stir butter into tequila mixture, a few pieces at a time, until a creamy sauce forms, about 2 minutes.
Stir in remaining ¼ teaspoon salt.
Add steak pieces plus drippings from plate; toss to coat steak in sauce.
Garnish with cilantro.
Serve immediately with tortillas and lime wedges, if desired.
