Preheat the oven to 350°F (175°C) and position a rack in the center. Coat three 8-inch round cake pans with baking spray, then line the bottoms with parchment paper for effortless cake removal.

In a medium bowl, whisk together the flour, cornstarch, baking powder, and kosher salt until evenly combined. Set the dry ingredients aside while preparing the batter.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter and granulated sugar on medium-high speed for about 3 minutes, or until the mixture becomes pale, light, and fluffy.

Lower the mixer speed and slowly drizzle in the vegetable oil. Increase the speed back to medium-high and continue mixing for about 1 minute until the mixture is smooth and fully emulsified.

Add the eggs and egg whites one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed to ensure an even batter. Blend in the vanilla extract and almond extract until incorporated.

Reduce the mixer to low speed. Add the flour mixture in three additions, alternating with the buttermilk. Begin and end with the dry ingredients, mixing only until no streaks of flour remain. Avoid overmixing to maintain a tender cake crumb.

Divide the batter evenly among the prepared cake pans, smoothing the tops with a spatula for even baking.

Bake for 35–40 minutes, rotating the pans halfway through the baking time. The cakes are done when the edges are lightly golden and a toothpick inserted into the center comes out clean.

Let the cakes cool in their pans for about 10 minutes, then carefully remove them. Peel away the parchment paper and transfer the layers to wire racks to cool completely, about 1 hour.

In the bowl of a stand mixer fitted with the whisk attachment, combine the softened cream cheese, mascarpone cheese, powdered sugar, and a pinch of kosher salt.

Beat on medium-high speed for 3–4 minutes until the mixture is silky smooth, creamy, and free of lumps.

Reduce the mixer speed to medium and slowly pour in the heavy whipping cream along with the vanilla extract.

Continue whipping for 1–2 minutes, stopping occasionally to scrape down the bowl, until the frosting becomes thick, fluffy, and forms stiff peaks. The finished frosting should be light enough to spread easily while still holding its shape.

In a small bowl, whisk together the raspberry jam with 3 tablespoons of hot water until it reaches a smooth, brushable consistency. This creates a flavorful syrup that adds moisture to each cake layer.

In a separate bowl, gently combine the blackberries, blueberries, raspberries, and sliced strawberries. Toss carefully to avoid crushing the delicate fruit.

If necessary, use a long serrated knife to trim the domed tops of the cooled cake layers, creating flat, even surfaces for stacking.

Spread a small spoonful of frosting onto the center of a cake stand or serving platter to prevent the cake from sliding during assembly.

Position the first cake layer on the stand. Brush the surface generously with the raspberry syrup, ensuring even coverage.

Spread approximately 1 cup of frosting over the cake layer, taking it nearly to the edges. Evenly scatter about 1 1/4 cups of the mixed berries across the frosting.

Repeat the same process with the second cake layer by brushing it with raspberry syrup, spreading another cup of frosting, and topping it with another 1 1/4 cups of berries.

Place the final cake layer on top with the trimmed side facing downward for the smoothest finish.

Cover the top and sides of the cake with the remaining frosting. Smooth the surface using an offset spatula, bench scraper, or the back of a butter knife for a polished appearance.

For added texture, lightly sweep an offset spatula or butter knife upward around the sides of the cake to create soft diagonal ridges.

Arrange the remaining mixed berries and the halved strawberries in an elegant crescent shape across one side of the cake's top.

If desired, gently warm the apricot jam until spreadable and lightly brush it over the fruit. This optional step gives the berries a beautiful glossy finish while helping them stay fresh and vibrant until serving.