Heat oven to 350°F (175°C).
Grease and flour a 9x13-inch pan or two 8-inch round cake pans. Line round pans with parchment for easy release.
In a large bowl, whisk eggs, oil, buttermilk, and vanilla until smooth.
Add the spice cake mix and cinnamon. Stir until just combined; do not overmix.
Fold in grated carrots, crushed pineapple, and any optional nuts or coconut.
Pour batter into prepared pan(s). Smooth the top.
Bake 25–30 minutes for a 9x13 or 22–27 minutes for 8-inch rounds. Use the toothpick test: it should come out with a few moist crumbs.
Cool in the pan for 10 minutes, then transfer rounds to a rack to cool completely. Let a 9x13 cool in the pan.
Beat cream cheese and butter until creamy and lump-free.
Add vanilla and salt. Gradually beat in powdered sugar until fluffy and spreadable.
Spread frosting over the cooled cake. For layers, add frosting between layers and on top and sides.
Top with chopped nuts or a light sprinkle of cinnamon if you like.
