01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, allowing overhang on sides for easy removal.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, nutmeg, and ginger until well blended.
03 - In a large bowl, whisk vegetable oil, granulated sugar, brown sugar, eggs, and vanilla extract until smooth and fully combined.
04 - Gently fold grated carrots into the wet mixture until evenly distributed.
05 - Add dry ingredients to wet mixture and stir until just combined. Fold in nuts if using.
06 - Spread batter evenly into prepared pan. Bake for 28 to 32 minutes, or until toothpick inserted in center comes out clean.
07 - Let cool completely in pan on wire rack before frosting.
08 - Beat cream cheese and butter together until creamy. Add powdered sugar, vanilla, and salt; beat until smooth and fluffy.
09 - Spread frosting evenly over completely cooled bars. Lift from pan using parchment paper, cut into 16 squares, and serve.