Combine the olive oil with soy sauce in a spacious bowl. Introduce the beef pieces, ensuring they are evenly coated, then let them rest at room temperature for 30 minutes or refrigerate for a minimum of 1 hour, stirring occasionally.
Warm a large, heavy-bottomed skillet (preferably cast-iron, 12-inch) over medium-high heat until thoroughly hot, about 2 minutes. Remove the steak from the marinade using a slotted spoon and arrange in a single layer in the pan, shaking off any excess. Sear without overcrowding, in batches if necessary, for 5 to 6 minutes, turning pieces only when they detach effortlessly from the skillet.
Once all the steak is browned, return any batches to the pan along with accumulated juices. Withdraw from heat, incorporate the butter and garlic, and stir briskly to coat the meat and meld flavors, about 1 minute.
Plate the steak bites, spooning over the pan's remaining sauce. Finish with a scattering of parsley, a light sprinkling of salt, and freshly cracked pepper. For convenience, skewer with toothpicks if serving as finger food.