Season both sides of the duck breasts with salt and pepper. Cover with 3 tablespoons maple syrup in a shallow dish. Cover and set in the refrigerator for 12 to 24 hours to marinate.

Preheat an electric smoker to 225 degrees F (110 degrees C) with maple wood chips according to the manufacturer's instructions.

Remove the duck breasts from the marinade and thoroughly rinse them. Using paper towels, pat dry. Cut shallow cross-hatch patterns into the skin of the duck breast with a sharp knife; do not cut all the way through to the flesh. 2 tablespoons maple syrup, brushed on.

Place the duck breasts, skin side up, on the smoker's grill rack. Underneath, place a drip pan. Smoke at 225°F (110°C) for 3 to 4 hours, adding more maple wood chips as needed to keep the smoke going, until duck breasts achieve an internal temperature of 160°F (70 degrees C). Allow to come to room temperature before slicing. Keep chilled until ready to serve.