Place the steaks on a small baking tray and transfer them to the freezer. Let sit for 30 minutes, to freeze the exterior.
Preheat oven to 70 degrees C (160 F) or the lowest temperature on your oven. Place a baking rack on top of a baking tray.
Heat a heavy skillet (*see footnote 2) over medium high heat until hot. While preheating the skillet, brush oil onto both sides of the steaks.
Brown the steaks one at a time, 30 seconds per side, until all of each surface turns dark brown. Immediately transfer each steak onto the prepared baking rack once browned.
Insert the oven-safe probe of the digital thermometer from the side, into the thickest part of the thinnest steak. Set the alarm to the desired temperature - 55 C / 133 F for medium rare, 54 C / 129 F for rare, or 62 C (144 F) for medium.
Bake until the steak reaches the set temperature. Baking time may vary greatly, depending on the thickness of the steak and on your oven. (*see footnote 3) The residual heat of the meat will continue to cook it after the steaks come out of the oven.
If your thermometer does not have an alarm setting, stay close to the oven and check the temperature frequently once the temperature is within about 10 C / 18 F of the target temperature, to avoid overcooking.
Combine all the ingredients for the ponzu dressing in a small bowl. Mix well and set aside.
After removing the steak from the oven, brush with melted butter and season generously with sea salt. Slice the steak into bite sized pieces, garnish with cilantro leaves, and serve immediately with the ponzu sauce for dipping.