Make a cut lengthwise down the centre of the pork, cutting to, but not through, the bottom. Open the butterflied portions, laying pork flat. Starting from the centre of each half, slice lengthwise down the centre of the pork, cutting to, but not through, the bottom. Unfold pork so it lies flat. Place plastic wrap over meat and flatten to 3/4-inch thickness using a meat mallet or rolling pin.