How to Make It
Tightly twist a 3-foot length of aluminum foil. Shape foil into a zigzag across bottom of a 5- or 6-quart round or oval slow cooker to create a rack, and add 1 cup water. Place lid on slow cooker, and set on LOW.
Whisk together sugar, flour, and salt in a large bowl, and set aside.
Beat egg whites and cream of tartar in a medium bowl with an electric mixer on medium speed until foamy, about 1 minute. Gradually increase speed to high, and beat until stiff peaks form, about 1 more minute. Beat in vanilla.
Sift sugar mixture over egg whites and fold gently to combine. Transfer batter to a metal 9- x 5-inch loaf pan, and smooth top.
Place pan in slow cooker, and press into foil rack to ensure pan is level. Drape a clean kitchen towel over slow cooker, and place lid over towel. Pull towel taut under lid. Start Cook on LOW until a wooden pick inserted in center comes out clean, about 2½ hours. Start checking cake around 2 hours and every half hour after that as slow cookers can vary greatly. Remove pan from slow cooker and invert onto a wire rack, and cool completely, about 30 minutes. Run a knife around edges to separate cake from pan, and invert onto a serving plate. Serve with Macerated Strawberries and whipped cream.