Preheat oven and place rack in the middle of the oven. Spray a loaf pan with nonstick baking spray, then set aside.
Sift together flour, baking powder, ground cinnamon, and Kosher salt, then set aside.
To grind walnuts, either place in the bowl of a food processor and pulse until a fine powder or place in a sealed Ziplock bag and smash with a rolling pin. Place crushed walnuts in a small bowl with raisins, then toss with remaining flour and set aside.
Carefully fold the flour mixture into the wet ingredients until just combined. Add finely ground nuts, raisins, and grated carrots, then fold once more until evenly distributed.
Pour batter into prepared loaf pan, then bake until evenly browned and a cake tester inserted into the middle of the loaf comes out clean with few crumbs.
Spread a generous even layer of cream cheese frosting on top of the cooled carrot cake loaf.