Preheat oven to 350 degrees F. Spray standard loaf pan with cooking spray and line with parchment paper, leaving 2-inch overhang on sides.

In large mixing bowl, combine flour, pumpkin pie spice, baking powder, baking soda and salt.

In separate medium bowl, combine oil, eggs, sugar, buttermilk and vanilla.

Add wet ingredients to dry and stir until combined. Fold in carrots and walnuts.

Pour batter into prepared pan. Bake for 50 to 60 minutes, until firm and a toothpick inserted into center comes out clean.

Cool for 5 minutes in pan, then transfer cake to wire rack and cool completely, removing parchment paper.

Meanwhile, make frosting. In mixing bowl with electric mixer or stand mixer, beat cream cheese and butter until fluffy. While mixing, add powdered sugar, then vanilla and salt, mixing until uniform.

When the loaf is fully cooled, cover with frosting.