Creating a moist and delicious Carrot Cake with Buttercream Frosting is easy when you follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
2. Mix Dry Ingredients: In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, nutmeg, and salt until well combined.
3. Combine Wet Ingredients: In another bowl, mix the vegetable oil and eggs until thoroughly blended. Add the grated carrots and crushed pineapple, if using, and stir to combine.
4. Combine Mixtures: Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Be careful not to overmix.
5. Add Nuts: If desired, fold in the chopped walnuts or pecans for added texture.
6. Pour Batter: Divide the batter evenly between the prepared cake pans.
7. Bake: Place the pans in the preheated oven and bake for 30-35 minutes. The cakes are done when a toothpick inserted in the center comes out clean.
8. Cool: Remove the cakes from the oven and let them cool in the pans for about 20 minutes. Then, transfer the cakes to wire racks to cool completely.
9. Prepare the Buttercream Frosting: In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, mixing until well combined. Add the vanilla extract and enough heavy cream to achieve your desired consistency.
10. Assemble the Cake: Once the cakes are completely cool, place one layer on a serving plate. Spread a generous amount of buttercream frosting on top. Add the second layer and frost the top and sides of the cake.