1 (15.25-ounce) box lemon cake mix
1 (3.4-ounce) box instant lemon pudding mix
2 cups milk (whole or 2%)
Lightly grease the inside of a 4- to 6-quart slow cooker with butter or nonstick spray.
In a large mixing bowl, whisk together the dry lemon cake mix and the dry instant lemon pudding mix until well combined.
Pour in the milk and stir until the batter is smooth and no dry streaks remain. The mixture will be thicker than regular cake batter but still pourable.
Pour the batter into the prepared slow cooker and spread it out evenly with a spatula.
Cover the slow cooker with the lid. Cook on LOW for 2.5 to 3.5 hours, or until the edges are set and the center is mostly set but still a little soft and jiggly. A toothpick inserted near the edge should come out clean, while the center will be more spoonable, like a soft pudding cake.
Once done, turn off the slow cooker and let the cake rest, covered, for about 10–15 minutes. This helps it set just enough while staying warm and tender.