Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, and salt.

In another bowl, combine the oil and eggs. Beat until well blended.

Add the grated carrots and crushed pineapple to the wet ingredients, mixing well.

Gradually add the dry ingredients to the wet mixture, stirring until just combined. Fold in the walnuts if using.

Divide the batter evenly between the prepared pans.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

In a mixing bowl, beat the softened cream cheese and butter together until smooth.

Gradually add the powdered sugar and vanilla extract, beating until creamy and well combined.

Once the cakes are completely cooled, place one layer on a serving plate.

Spread a layer of frosting on top of the first cake layer.

Place the second cake layer on top and frost the top and sides of the cake.

Decorate with additional walnuts if desired.