Preparation Steps

• Preheat the oven to 340°F (171°C) with a rack in the center. Line three 8-inch round cake pans with parchment and lightly grease the sides. If you use bake-even strips, soak and attach them now.
• Massage the lemon zest into the granulated sugar with your fingertips until the sugar is fragrant and pale yellow. Measure out and reserve 1 teaspoon of this zest for the frosting; keep the rest with the sugar for the batter.
• In a medium bowl, whisk together the flour, baking powder, and salt; set aside.
• In a large bowl, whisk the eggs, oil, and sour cream until completely smooth. Whisk in the buttermilk, lemon juice, and lemon oil. Sprinkle in the lemon-sugar mixture and whisk until dissolved and glossy.
• Switch to a spatula and fold the dry ingredients into the wet in two additions, mixing just until no dry streaks remain. Let the batter rest for 5 minutes to hydrate for a finer crumb.
• Meanwhile, spread the raspberries on a plate and freeze for 10 minutes (this helps minimize bleeding). Toss the chilled berries with 1 tablespoon flour, then gently fold them into the batter.
• Divide the batter evenly among the pans (about 1 2/3 cups per pan). Tap each pan once on the counter to release large air bubbles. Bake 22–26 minutes, rotating once halfway, until the centers spring back and a toothpick comes out with a few moist crumbs.
• Cool cakes in pans for 10 minutes, then invert onto racks, peel off parchment, and cool completely before frosting.
• For the mascarpone whipped cream: Chill a mixing bowl and whisk attachment in the freezer for 10 minutes. In the chilled bowl, whip the heavy cream with 1/4 cup of the powdered sugar to soft peaks; set aside. In a separate bowl, beat mascarpone with the remaining 1/2 cup powdered sugar, vanilla bean paste, a pinch of salt, and the reserved 1 teaspoon lemon zest until smooth. On low speed, blend the whipped cream into the mascarpone in two additions just until thick and spreadable—do not overmix.
• Assembly: Level any domed tops. Place one cake layer on a board. Using a piping bag fitted with a large round tip, pipe a 1/2-inch dam of frosting around the edge. Spoon about 1/3–1/2 cup lemon curd inside the dam and spread gently. Top with the second layer and repeat with another dam and curd. Place the final layer on top, cut side down.
• Apply a thin crumb coat of frosting over the top and sides and chill for 15 minutes. Finish with a thicker coat, smoothing or adding rustic swirls as you like. Garnish with fresh raspberries and thin lemon slices.
• Refrigerate the cake for at least 30 minutes to set before slicing. For clean slices, warm a long knife under hot water and wipe it dry between cuts. Store the cake covered and refrigerated for up to 3 days.