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