Preheat oven to 350°F (175°C) with racks in lower third and center positions. Grease two 9-inch round pans with butter or flax oil and line with parchment paper.
Whisk together flour, cinnamon, nutmeg, ginger, baking powder, and salt in one bowl. Set aside.
In another bowl, cream butter, brown sugar, and granulated sugar until light and fluffy. Add eggs, one at a time, incorporating fully after each addition. Stir in vanilla extract and grated carrots.
Alternate adding the dry ingredients and buttermilk to the wet ingredients. Fold in walnuts just before mixing in raisins or pineapple (optional). The batter should be slightly lumpy but smooth overall.
Cover pans with lightly greased parchment paper and bake for 30-35 minutes until a toothpick inserted in the center comes out clean. Cool completely before frosting.
Blend soaked cashews, vegan cream cheese, powdered sugar, lemon zest, and vanilla until silky. For indulgence, add melted chocolate or orange blossom water.
Crumb-coat the first layer first to contain any escaping crumbs. Chill both layers before assembling.