Pour 1 in. water into a 6-qt. slow cooker. Layer two 24-in. pieces of foil; roll up lengthwise to make a 1-in.-thick roll. Shape into a ring; place in slow cooker to make a rack.
Whisk together first 7 ingredients. Transfer to a greased 2-qt. baking dish; set aside. Fold an 18x12-in. piece of foil lengthwise into thirds, making a sling. Use the sling to lower the baking dish onto foil rack, not allowing sides to touch slow cooker. Cover slow cooker with a double layer of white paper towels; place lid securely over towels.
Cook, covered, on low 6-7 hours or until a thermometer reads 160°. Remove baking dish from slow cooker using sling. Top with crushed gingersnap cookies and, if desired, whipped cream.
Test Kitchen variations
This recipe can also be prepared directly in a smaller slow cooker. Whisk together first 7 ingredients. Transfer to a greased 3-qt. slow cooker. Cook, covered, on low until a thermometer reads 160°, 6-7 hours. Top with crushed gingersnap cookies and if desired, whipped cream.