Remove any scuffed cabbage leaves. Then peel off 8 of the outermost, larger green leaves and remove the tough core with a pairing knife. Have a ready a bowl of ice water. Bring a pot of salted water to a boil and blanch the 8 cabbage leaves until tender but not falling apart, 2 to 3 minutes. Immediately shock the leaves in the ice water to stop the cooking. Dry the leaves thoroughly between paper towels and set aside.
Lay one very long piece of plastic wrap on the counter. Lay 2 cabbage leaves on the plastic wrap, overlapping just a bit. Dot the cabbage with 1 teaspoon of the truffle butter. Season one breast with salt and set it on one edge of the cabbage leaves and fold the excess cabbage over the breast. If there is excess, cut with kitchen shears. Continue to wrap the pheasant breast tightly with the plastic wrap, so the plastic goes twice around; there should be extra wrap on the ends to tie off into a knot. Repeat with the remaining breasts. Seal the breasts in sous vide bags. Using an immersion circulator to keep a constant temperature, cook the breasts at 144 degrees F for 30 minutes.
Melt the butter in a saucepan set over low heat. Add the squash and cook until almost tender, not allowing it to brown at all. Add the water, cover the pan, and simmer until almost no water remains. Use a hand blender to blend until smooth. Season to taste with salt.
To serve, slice the breasts on the bias, three slices per breast. Spoon the butternut squash puree onto a plate. Set the breast over the puree and finish with shaved truffles and sea salt.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.