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 incorporated.
04 - Fold grated carrots into the wet mixture until evenly distributed throughout.
05 - Add dry ingredients to wet mixture and stir until just combined. Fold in nuts if using.
06 - Transfer batter to prepared pan and spread evenly into corners using an offset spatula.
07 - Bake for 28-32 minutes or until toothpick inserted in center comes out clean. Avoid overbaking.
08 - Let bars cool completely in pan on wire rack before frosting.
09 - Beat softened cream cheese and butter together until creamy. Add powdered sugar, vanilla, and salt; beat until smooth and fluffy.
10 - Spread frosting evenly over cooled bars. Lift from pan using parchment paper overhang, cut into 16 squares, and serve.