Preheat your oven to 350°F (180°C). Grease a 9×13 inch baking pan and set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
In a large bowl, beat together the sugar, eggs, and oil until well combined. Add the dry ingredients to the wet ingredients and stir until just combined. Stir in the grated carrots and nuts, if using.
Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely.
To make the frosting, beat together the cream cheese and butter until smooth. Add the confectioners’ sugar and vanilla and beat until well combined and smooth. Spread the frosting over the cooled cake and serve.