How to Make It
Tightly twist a 3-foot length of aluminum foil to create a rack for slow cooker. Shape foil into a zigzag across bottom of 5- to 6-quart round or oval slow cooker, and add water. Place lid on slow cooker, and set on LOW. Grease and flour an 8-inch round cake pan.
Beat oil, brown sugar, granulated sugar, buttermilk, egg, and vanilla with an electric mixer on medium speed until smooth and thickened, about 1 minute. Whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl. Sprinkle half of flour mixture over oil mixture, and fold gently to combine. Repeat with remaining flour mixture. Fold in carrots and, if desired, nuts. Pour batter into prepared pan, smoothing 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. Cook on LOW until a wooden pick inserted in center comes out clean, about 2 hours. Cool cake in pan on a wire rack until just cool enough to handle, about 20 minutes. Place a piece of parchment paper on a wire rack, and invert cake onto parchment. Cool completely, about 30 minutes.
Split cake into 2 even layers using a serrated knife. Place 1 layer on a cake plate, and top with about 1 cup of the Cream Cheese Frosting, spreading frosting to edges, but not over the side of the cake. Top with second layer, and spread remaining frosting over top, leaving sides exposed.