1. Preheat the oven to 350F. Grease and flour a 13x9-inch baking pan.
2. In a large bowl using an electric mixer, beat the sugar, oil and eggs until light. Stir in the flour, cinnamon, baking soda and salt until well combined. Gently stir in the carrots and sunflower seeds (if using).
3. Pour the batter into the prepared pan. Bake until a toothpick inserted in the center comes out clean, 25 to 30 minutes. Transfer the pan to a wire rack and cool completely.
4. Spread the bars with enough of the frosting to cover completely. Drag a fork through the frosting on a diagonal to decorate. Cut into 2-inch squares and remove from the pan.
5. Divide the remaining frosting among 2 small bowls; tint green and orange. Fill a pastry bag fitted with a small star tip with the green frosting; pipe carrot tops on top of each square. Fill a clean bag fitted with a small writing tip with orange frosting; pipe small carrot shapes below each green top.
Tip
Be sure to clean the fork often when dragging it through the frosting. If the frosting is too soft to create the design, place the pan in the fridge and chill for 10 minutes and then try again.