01 - Preheat oven to 350°F. Grease and line an 8x8-inch baking pan with parchment paper.
02 - Blend fresh spinach in a food processor until finely chopped.
03 - Whisk sugar, vegetable oil, eggs, and vanilla extract in a large bowl until smooth. Stir in chopped spinach.
04 - Whisk together flour, baking powder, baking soda, salt, and cinnamon in a separate bowl.
05 - Add dry ingredients to wet mixture and stir gently until just combined. Fold in walnuts if using.
06 - Pour batter into prepared pan, smoothing the top. Bake 25 to 30 minutes until a toothpick inserted comes out clean. Cool completely in the pan.
07 - Beat cream cheese and butter until creamy. Gradually add powdered sugar, vanilla extract, and salt, mixing until smooth.
08 - Spread frosting evenly over cooled bars.
09 - Cut into 16 bars and serve.