Prick the sweet potatoes all over with a fork. Place on paper towels in the microwave and cook on high power for 15 to 20 minutes or until tender, turning after 10 minutes. Remove from microwave and cool slightly. When potatoes are just cool enough to handle, slice each one in half and scoop the flesh into a large bowl. Mash well with a potato masher (you should have about 4 cups).
Coat a 2-quart square baking dish with nonstick cooking spray; set aside. To the sweet potato flesh, add the orange zest, orange juice, 1/4 cup of the sugar, 3 tablespoons of the butter, eggs, maple syrup, 1/2 teaspoon of the salt, half of the cinnamon, all of the nutmeg, and the ginger. Stir to combine. Spoon mixture into prepared dish, spreading evenly. Cover and chill until baking time or up to 24 hours.
Place the graham crackers in a quart-size resealable plastic bag. Seal and use a rolling pin to crush into fine crumbs (you should have 1 cup crumbs). Add remaining butter, sugar, cinnamon, and salt to crumbs. Seal bag and shake and knead to combine. Chill crumbs until baking time or up to 24 hours.
If chilled, allow sweet potato mixture to stand at room temperature for 1 hour before baking. Preheat oven to 350°F. Sprinkle the crumbs over the sweet potato mixture. Bake for 35 minutes or until heated through (165°F) and crumbs are golden.