Preheat oven to 350°.
Score the duck breast skin without cutting into the breast, cut in a criss cross pattern.
Season the duck breast on both sides with sea salt and pepper. Heat a sauté pan over medium high heat until hot and sear the breast skin side down, do not put oil or butter into the pan, you’ll have plenty of rendered duck fat shortly. When the breast is nicely browned turn and cook the other side, turn the heat down on the pan if needed.
Remove breast to a small baking sheet lined with parchment, a baking pan will do as well. Do not use a pan that’s too big or the fat will burn as it’s rendered, place a probe thermometer into the center of the breast and cook until temperature reaches 135°. Remove from oven and let rest while you finish the sauce.
Combine orange juice, sugar, orange zest, and half of the Grand Marnier in a small saucepan. Bring to a boil, uncovered, over medium-high heat. Reduce heat to medium and simmer until the liquid is reduced to 2–3 tablespoons. Remove the pan from the heat and whisk in butter, a few pieces at a time, until thoroughly blended, and then whisk in the remaining Grand Marnier.
Slice the duck breasts on an angle and serve with the Grand Marnier Sauce, top with some orange peel or zest.