Preheat your oven to 350 degrees F. Lightly grease a 13 x 18 x 1-inch baking pan. Set aside.

In a large mixing bowl, combine the cake mix and eggs. Beat with an electric mixer until smooth and fully blended. Add the lemon pie filling and gently mix just until incorporated.

Spread the batter evenly into the prepared pan, smoothing the top with a spatula. Bake for about 15 to 18 minutes, or until the center springs back lightly when touched and a toothpick inserted in the middle comes out clean. Let the cake cool completely in the pan on a wire rack.

For the frosting, place the softened cream cheese and butter in a bowl and beat until creamy and smooth. Gradually add the confectioners' sugar, mixing until fully combined. Stir in the vanilla extract and a pinch of salt. Add milk one tablespoon at a time until the frosting reaches a smooth, spreadable consistency.

Once the cake has cooled, spread the frosting evenly over the top. If you like, garnish with quartered lemon slices. Store the cake covered in the refrigerator until ready to serve.