My mother found this retro recipe in a dusty cookbook and it blew us away.

Prepare the slow cooker: Lightly coat the bottom and about 2 inches up the sides of a 5- to 6-quart slow cooker with nonstick cooking spray or a thin layer of oil. If you’d like easier removal, you can also line the bottom with a strip of parchment paper that hangs up the sides like a sling.

Mix the cake batter: In a large mixing bowl, whisk together the white cake mix and the instant key lime (or lime) pudding mix until evenly combined and no lumps of pudding mix remain.
Add wet ingredients: Pour in 1 cup of key lime or lemon-lime soda and 1/3 cup of neutral oil. Stir with a spoon or spatula just until the batter is smooth and everything is incorporated. The batter will be slightly thicker than regular cake batter and a pale green color if your pudding mix is tinted.

Transfer to slow cooker: Pour the batter into the prepared slow cooker and spread it into an even layer with a spatula, smoothing the top so it cooks evenly.
Cook the cake: Cover the slow cooker with its lid. Cook on HIGH for 2 to 2 1/2 hours, or on LOW for 3 1/2 to 4 hours, until the cake is set in the center. You’ll know it’s done when the top looks dry, the edges pull slightly away from the sides, and a toothpick inserted into the middle comes out with just a few moist crumbs and no wet batter.

Cool slightly: Turn off the slow cooker and remove the lid, being careful of the steam. Let the cake cool in the slow cooker for at least 20 to 30 minutes. This helps it set up so it slices cleanly and keeps the crumb nice and moist.
Make the glaze (optional but highly recommended): In a small bowl, whisk together 1 cup powdered sugar with 2 tablespoons key lime or lemon-lime soda until smooth and pourable. If it’s too thick, add up to 1 more tablespoon soda, a little at a time, until it reaches a thick but drizzleable consistency.

Glaze the cake: Either leave the cake right in the slow cooker or gently lift it out using the parchment sling (or by cutting into large pieces and transferring to a plate). Drizzle the glaze generously over the warm cake so it soaks in a bit and forms a shiny white layer on top.
Serve: Slice the cake into squares or wedges. Serve warm or at room temperature, with any extra glaze spooned over the top. Store leftovers covered at room temperature for up to 2 days or in the refrigerator for up to 4 days.
Variations & Tips