Preheat oven to 350 degrees F. Grease a 13x9 inch baking pan thoroughly.
In a large bowl, combine lemon cake mix and instant lemon pudding mix. Stir until evenly distributed.
Add eggs, sour cream, vegetable oil, water, lemon zest, and lemon extract to dry ingredients. Beat on medium speed for 2 minutes until smooth.
Pour batter into prepared pan and spread evenly. Bake for 30 to 35 minutes until a toothpick inserted in center comes out clean.
Place pan on wire rack and let cake cool completely. Meanwhile, beat cream cheese and butter until light and fluffy.
Gradually add powdered sugar one cup at a time, mixing well. Stir in lemon extract for frosting.
Spread frosting evenly over cooled cake. Garnish with fresh lemon slices if desired. Serve and enjoy.