Place the duck breasts skin-side down on a cutting board and trim any excess skin that hangs over the sides of the breasts. Flip the breasts over and score the skin with four parallel, diagonal cuts. Dust the breasts on both sides with the five-spice powder.
Heat a cast-iron pan over medium-low heat. Place duck breasts in the pan, skin-side down, and cook until the fat has melted and the skin is golden brown, about 10 minutes. Flip the breasts over and continue to cook for about 10 minutes for medium doneness. Remove the duck to a cutting board and let rest for 5 minutes (this ensures the duck will be plump and juicy). To serve, cut the breasts into 1/4-inch-thick slices and spread out in a shingle pattern on a large platter.