Preheat grill to medium-high (if you are using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 3 to 4 seconds). Stir butter, jalapeno, and garlic in a bowl; set aside.
Season both sides of steak with salt and pepper. Grill over hotter part of grill, covered, flipping once, until browned, 3 to 4 minutes per side. Slide to cooler part of grill; grill, covered, flipping once, until medium-rare, 7 to 8 minutes more per side. Remove from grill, and let rest 10 minutes. Serve with jalapeno butter.
Filet mignon can be prepared this way as well. Sear two 8-ounce steaks over direct heat for three minutes per side; grill over indirect heat until medium-rare, five to six minutes more per side.