Position a rack in the center of the oven and preheat oven to 350°F (175°C). Grease two 9-inch round cake pans and line bottoms with parchment; grease parchment and lightly dust pans with flour, tapping out excess.

Dry ingredients

In a large bowl, whisk together 2.5 cups all-purpose flour, 1 cup granulated sugar, 0.5 cup light brown sugar, 1 teaspoon baking powder, 1 teaspoon baking soda, 1/2 teaspoon fine sea salt, 2 teaspoons ground cinnamon, 1/4 teaspoon ground nutmeg, and 1/4 teaspoon ground ginger until evenly combined.

Wet ingredients

In a separate medium bowl, whisk 3 large eggs lightly, then add 1 cup vegetable oil, 1/2 cup buttermilk and 2 teaspoons vanilla extract. Whisk until combined.

Combine batter

Make a well in the dry ingredients and pour in the wet mixture. Stir with a sturdy spatula or wooden spoon until just combined and no dry streaks remain. Fold in 3 cups packed grated carrots, 3/4 cup chopped walnuts (if using), and 1/2 cup raisins (if using) until evenly distributed. Do not overmix; batter will be thick.

Bake

Divide batter evenly between the prepared pans (use a scale or measure by eye). Smooth the tops. Bake on the center rack for 28–34 minutes, rotating pans halfway through, until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter) and the tops spring back when lightly pressed.

Cool

Remove cakes from oven and let pans rest on a wire rack for 10–15 minutes. Run a knife around the edges, invert onto the rack, remove parchment, and allow layers to cool completely (about 1 hour) before frosting to avoid melting the frosting.

Cream Cheese Frosting

Prepare frosting

Using a stand mixer fitted with the paddle or a hand mixer, beat 1/2 cup softened unsalted butter on medium speed until smooth, about 30–45 seconds. Add 8 oz cream cheese (cold or slightly cool) cut into chunks and beat on medium-low until fully incorporated and smooth, scraping down the bowl as needed.

Sweeten and flavor

Gradually add 3 cups sifted confectioners' sugar, about 1 cup at a time, mixing on low speed until incorporated. After all sugar is added, increase speed to medium and beat until light and fluffy, about 1–2 minutes. Add 1 teaspoon vanilla extract, 1/8 teaspoon fine sea salt, and 1 teaspoon lemon zest (optional). Taste and adjust sweetness or salt as desired.

Chill if needed

If frosting is too soft to spread (warm kitchen), chill in the refrigerator for 15–20 minutes and briefly rewhip on low speed to restore spreadability.

Assemble & Finish

Level tops of cake layers with a serrated knife if domed. Place one layer on a serving plate or cake stand. Spread about one-third of the frosting evenly over the top (use an offset spatula). Place the second layer on top, aligning edges, and apply a thin crumb coat of frosting over the entire cake to trap crumbs. Chill the cake 15–20 minutes to set the crumb coat.

Use remaining frosting to finish the top and sides in a smooth or decorative pattern. Press toasted walnut halves or pieces into the sides or scatter on top (about 1/4 cup). Lightly dust with 1/4 teaspoon ground cinnamon if desired. Refrigerate the assembled cake for at least 30 minutes before slicing for cleaner cuts.

Bring slices to room temperature for 20–30 minutes before serving for best texture and flavor. Store leftover cake covered in the refrigerator for up to 4 days.