Preheat and Prep: Preheat oven to 350°F (175°C). Grease a 9x13-inch pan and lightly dust with flour.
Mix Batter: In a bowl, combine cake mix, lemon Jello, eggs, oil, and water. Beat until smooth, but don’t overmix.
Bake: Pour batter into the pan and bake 25-30 minutes, or until a toothpick comes out clean.
Cool: Let the cake cool in the pan for 15 minutes, then transfer to a cooling rack.
Glaze (Optional): Mix powdered sugar, lemon juice, and melted butter. Drizzle over warm cake.
Serve: Slice and enjoy as-is or with whipped cream, berries, or ice cream.