• Preheat your oven to 350°F (175°C). Grease and flour three 8-inch cake pans, or coat them with baking spray. For extra security, line the bottom of each pan with parchment paper to prevent sticking.
• In a large mixing bowl or the bowl of a stand mixer, combine the flour, sugars, baking soda, baking powder, and salt. Stir together until everything is evenly distributed. With the mixer on low, gradually add the softened butter, one tablespoon at a time, ensuring each piece is fully mixed in before adding the next. The texture should resemble slightly coarse sand once all the butter is incorporated.
• In a separate bowl, whisk together the buttermilk, eggs, and vanilla extract until smooth. Slowly pour this wet mixture into the dry ingredients, keeping the mixer on low speed. Stop occasionally to scrape down the sides of the bowl, making sure the batter is completely smooth and silky.
• Divide the batter evenly among the three prepared cake pans. Place them in the preheated oven and bake for 30-32 minutes, or until a toothpick inserted into the center comes out clean. Rotate the pans halfway through baking for even results. Once baked, allow the cakes to cool in the pans for 10-15 minutes before running a knife around the edges and inverting onto a cooling rack. Let the cakes cool completely before assembly.

Berry Filling

• In a small saucepan, whisk together the sugar and cornstarch. Add the mixed berries, water, and lemon juice, stirring to combine all ingredients.
• Place the saucepan over medium heat and cook slowly, stirring constantly until the berries release their juices and the mixture thickens to a jam-like consistency. Use a fork or potato masher to gently break up the berries as they cook. Be mindful that the filling will thicken further as it cools, so avoid overcooking it into a paste. Transfer the berry mixture to a heatproof container and allow it to cool completely before spreading it on the cake.

Chantilly Cream Frosting

• In a large bowl, combine the heavy cream, 1 ½ cups of powdered sugar, and vanilla extract. Use an electric mixer to beat the mixture until stiff peaks form. The whipped cream should hold its shape, though a slight fold at the peak is acceptable.
• In a separate bowl, blend the softened cream cheese, mascarpone, and the remaining ½ cup of powdered sugar. Beat briefly on low speed until smooth, but be careful not to overmix the mascarpone, as it can become grainy. Gradually fold the mascarpone mixture into the whipped cream and beat on low speed, then increase to high until the frosting is light and fluffy.

Cake Assembly

• Place the first layer of cooled cake on your serving dish. Pipe a ring of frosting around the edge to form a dam. Spread half of the cooled berry filling inside the ring. Add the second cake layer and repeat the process by piping another dam and adding the remaining berry filling.
• Top with the final layer of cake. Cover the entire cake with the Chantilly cream frosting, smoothing it evenly. If desired, use the leftover frosting to pipe decorative rosettes or swirls on top. Finish by garnishing the cake with fresh berries for a beautiful presentation.
• Store the cake covered in the refrigerator and enjoy!