Preheat the oven to 350 degrees F (175 degrees C). Lightly spray a sheet cake pan or a 10×15-inch glass casserole dish with cooking spray.
Sift together flour, baking powder, cinnamon, baking soda, and salt in a bowl.
Beat eggs in the bowl of a stand mixer until foamy. Slowly add in sugar and mix until incorporated. Beat in flour mixture alternately with the oil, beginning and ending with flour mixture. Fold in pineapple, carrots, coconut, and walnuts. Pour batter into the prepared cake pan.
Bake in the preheated oven for 45 minutes. Remove from oven and cool completely, about 30 minutes.
Combine softened butter and cream cheese in a bowl. Whip until fluffy using an electric mixer. Add sifted powdered sugar and vanilla extract. Mix until well incorporated and fluffy. Spread over the cooled carrot cake.
The powdered sugar needs to be sifted first, and then measured.
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