Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan and set aside.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, nutmeg, and salt.
In another bowl, combine the vegetable oil, eggs, grated carrots, crushed pineapple, walnuts (if using), and vanilla extract. Mix until well combined.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Do not overmix.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
Allow the bars to cool in the pan for about 15 minutes, then transfer to a wire rack to cool completely.
While the bars are cooling, prepare the cream cheese frosting. In a medium bowl, beat together the softened cream cheese and butter until smooth.
Gradually add the powdered sugar and vanilla extract, mixing until well combined. If the frosting is too thick, add milk one tablespoon at a time until the desired consistency is reached.
Once the bars are completely cooled, spread the cream cheese frosting evenly over the top. Cut into squares and serve.