1. Preheat the oven to 350 degrees F. Spray a 9 by 13-inch baking pan with vegetable spray; line it with a piece of parchment paper, leaving an 1-inch overhang on 2 sides and spray the paper.
2. Whisk together the flour, baking soda, salt and pumpkin pie spice in a medium bowl. In another medium bowl, whisk the eggs and sugars until pale; whisk in the oil, then the buttermilk and vanilla. Stir the wet ingredients into the dry ingredients and fold in the carrots. Pour the batter into the prepared pan and smooth the top.
3. Bake until a toothpick inserted in the center of the cake comes out with a few moist crumbs attached, 35 to 40 minutes. Cool completely on a wire rack. Using the parchment overhang, remove the cake from the pan. Frost the cooled cake with Lighter Creamy Frosting, if desired. Cut into 16 (2 by 3-inch) pieces and serve.
Lighter Cream Cheese Frosting:
In a medium bowl, stir together the cream cheese, confectioners' sugar, and vanilla.
Spread the frosting over the top and sides of the cake.