Preheat oven to 350°F (175°C) and grease a 9x13 inch baking pan.

In a large mixing bowl, combine the spice cake mix, eggs, vegetable oil, and water.

Mix on medium speed for 2 minutes until the batter is well combined.

Gently fold in the grated carrots and chopped walnuts using a spatula.

Pour the batter into the prepared baking pan and level the surface.

Bake for 30 to 35 minutes or until a toothpick inserted into the center of the cake comes out clean.

Remove from the oven and let the cake cool completely in the pan on a wire rack.

In a separate bowl, beat the softened cream cheese and butter until smooth and creamy.

Gradually beat in the powdered sugar and vanilla extract until the frosting is light and fluffy.

Spread the cream cheese frosting evenly over the cooled cake before serving.