Step 1: Preparing the Cake Layers

1. Preheat the oven to 350°F (176°C). Grease three 8-inch cake pans with non-stick spray and line the bottoms with parchment paper. This ensures easy removal.
2. Mix the dry ingredients. In a large mixing bowl, combine flour, sugar, baking powder, and salt. Stir to distribute evenly.
3. Prepare the wet ingredients. In another bowl, whisk together the milk, vegetable oil, vanilla extract, and eggs.
4. Combine wet and dry ingredients. Pour the wet mixture into the dry ingredients and mix until just combined.
5. Add the water slowly. Pour in the water gradually while mixing on low speed. This step ensures a smooth, lump-free batter.
6. Divide and bake. Evenly distribute the batter among the three cake pans. Bake for 24-28 minutes, or until a toothpick inserted in the center comes out clean.
7. Cool completely. Let the cakes cool in the pans for about 5 minutes before transferring them to a wire rack.

Step 2: Making the Berry Filling

1. Blend the berries. Use a food processor or blender to puree the mixed berries with 3 tablespoons of water. You should get about ⅔ cup of puree.
2. Cook the filling. In a saucepan, combine sugar and cornstarch, then stir in the berry puree.
3. Thicken the mixture. Cook over medium heat, stirring continuously, until it thickens and starts to boil. This should take about 8-10 minutes.
4. Cool the filling. Once thickened, remove from heat and let it cool completely before using.

Step 3: Making the Mascarpone Frosting

1. Whip the cream. In a large bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract on high speed until soft peaks form.
2. Add mascarpone cheese. Gently fold in the mascarpone and continue whipping until the mixture forms stiff peaks. Be careful not to overwhip, as it can become grainy.

Step 4: Assembling the Cake

1. Level the cakes. Use a serrated knife to trim any domed tops for even stacking.
2. Create a frosting border. Place the first cake layer on a serving plate and pipe a thick border of frosting around the edges.
3. Add berry filling. Spread half of the berry filling inside the frosting border.
4. Layer the frosting. Spread a thin layer of mascarpone frosting over the berry filling.
5. Repeat the process. Place the second cake layer and repeat the filling and frosting steps.
6. Top with the final layer. Frost the entire cake with the remaining mascarpone frosting, smoothing it out with an icing spatula.
7. Decorate. Add fresh berries and use a cake comb to create a striped texture if desired.