Pierce skin of sweet potatoes in several spots with a fork.
Cook sweet potatoes in microwave oven until just tender, 12 to 15 minutes. Refrigerate sweet potatoes until cooled completely, at least 20 minutes.
Preheat oven to 350 degrees F (175 degrees C). Prepare a shallow 1 1/2-quart baking dish with cooking spray.
Peel sweet potatoes and cut into cubes; spread into prepared baking dish.
Mix sugar, cornstarch, salt, and ginger together in a glass measuring cup; pour orange juice into the cup while stirring with a whisk.
Heat orange juice mixture in microwave oven for 1 minute, stir, and continue to heat until thickened and smooth, 30 to 60 seconds more; add butter and stir until melted. Pour the liquid over the sweet potatoes.
Bake in preheated oven until sweet potatoes are heated through, 15 to 20 minutes.