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

In a large bowl, whisk together flour, baking soda, cinnamon, and salt.

In a separate bowl, combine sugar, oil, and eggs. Mix well.

Add the wet ingredients to the dry ingredients and mix until just combined.

Stir in carrots and nuts (if using).

Pour batter into prepared pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Let cool completely before frosting.

In a large bowl, beat cream cheese and butter until smooth.

Gradually add powdered sugar and vanilla extract and beat until smooth and creamy.

Spread frosting evenly over cooled bars. Cut into squares and serve.