Bring the sweet potatoes to a boil in a large pot of salted water. Cook until just tender when pierced with a knife, 10 to 15 minutes. Drain and cool slightly.
Combine the butter, molasses, orange zest and cinnamon in a bowl. Season with salt and pepper. Mash with a fork to blend.
Heat a grill to medium-high. Cut each sweet potato into 6 to 8 wedges; brush with canola oil and season with salt and pepper. Grill the wedges until golden brown, about 3 minutes per side. Remove the sweet potatoes to a platter; immediately brush them with the orange-molasses butter and sprinkle with the green onions. Serve.
The orange-molasses butter can be made two days ahead and stored, covered, in the refrigerator. Just remove it from the refrigerator 20 minutes before using.