Preheat the oven to 350 degrees. Spray a 9x13 baking pan with nonstick spray. Combine the cake mix, milk, vegetable oil, sour cream, eggs, lemon juice, and lemon zest in a medium bowl. Stir with an electric mixer until well combined. Pour into the prepared pan and spread into an even layer. Bake for 25-35 minutes, or until a toothpick inserted in the center comes out with only a few crumbs. Remove from the oven and let cool for 5-10 minutes.

While the cake cools, combine the butter, lemon juice, and milk in a small saucepan over medium heat. Whisk until melted and smooth, but not boiling. Remove from heat, add the salt, powdered sugar, and lemon zest, and whisk until smooth. While the cake is still warm, pour the warm frosting evenly over the top. Allow the frosting to set, then top with lemon slices if desired.