Preheat oven to 350°F. Place parchment paper or silicone baking mats on two large baking sheets.
In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt. Set aside.
In a large bowl, use a hand mixer to beat together the sugar and softened butter until smooth and creamy. Add the eggs and beat until combined.
Add half of the flour mixture, beat on low just until combined. Add half of the buttermilk and beat until just combined. Repeat with the remaining flour mixture and buttermilk.
Stir in the grated carrots and chopped walnuts. Use a 1-oz ice cream scoop (about 2 tablespoons) to drop cake batter onto the baking sheets, leaving 2 inches in between each mound of batter.
Bake for 12–14 minutes, or until the top of the cakes no longer appear wet. Let cool for 5 minutes on the baking sheet, then transfer cakes to a wire rack to cool completely.
In a large bowl, use a hand mixer to beat together the softened cream cheese and softened butter until smooth. Beat in the vanilla, orange juice, and orange zest.
Add 1 cup of powdered sugar and beat on low until combined. Repeat with the remaining powdered sugar, ½–1 cup at a time until all the powdered sugar is incorporated.
Spread approximately 2 tablespoons of the cream cheese icing on the flat side of a cake. Top with another cake, flat side down. Repeat with remaining cakes to create 12 whoopie pies.