• Gather all ingredients and equipment before starting.

For the strawberry & raspberry filling:

• Whisk the cornstarch into about half of the water until smooth and set aside.
• Place half of the diced strawberries and all the raspberries into a large saucepan with the remaining water, sugar, lemon juice, and vanilla. Purée the mixture with an immersion blender until smooth.
• Bring the puréed berries to a boil over medium heat, then reduce to a simmer for 2–3 minutes.
• Stir the cornstarch slurry into the simmering berries and continue cooking, stirring, until the mixture thickens; let it bubble for about 2 more minutes to reach a custard-like consistency.
• Remove from the heat, let cool briefly, then fold in the remaining diced strawberries. Press plastic wrap directly onto the surface to prevent a skin and chill until completely cold. (This can be prepared a day ahead.)

For the cake layers:

• Preheat the oven to 170°C (340°F). Lightly grease four 20 cm round silicone baking molds and set them aside.
• Separate the eggs. Put the egg whites and a pinch of salt into the bowl of a mixer and whip until they become foamy.
• With the mixer running, add half of the sugar (about 100 g) gradually and continue beating on high until the whites are thick, glossy, and hold volume (around 2 minutes after the sugar is incorporated).
• In a separate bowl, beat the egg yolks with the remaining sugar (about 100 g) and vanilla until pale and slightly fluffy, about 2–3 minutes.
• Sift together the flour, baking powder, and the 1/4 teaspoon salt. Add the dry ingredients to the yolk mixture in batches, folding with a rubber spatula.
• Fold the whipped egg whites into the batter in several additions. Start by loosening the batter with a gentle but slightly more vigorous fold, then finish with careful folds to retain volume.
• Divide the batter evenly between the prepared molds and bake for 20–23 minutes, rotating the molds halfway through baking. When done, invert the molds onto wire racks and remove the silicone once cooled slightly.

For the mascarpone cream:

• Combine cold mascarpone, confectioners' sugar, and vanilla in a large bowl and beat until smooth and homogenous.
• Add the cold heavy cream and begin whisking slowly, then increase speed and whip until the mixture forms firm peaks, about 2–3 minutes.
• Transfer the cream to a piping bag fitted with a star tip and set aside until assembly.

Assembly and decoration:

• Place one cake layer on a serving plate. Pipe a small amount of mascarpone cream onto the layer and spread it gently to an even thickness using an angled spatula.
• Pipe a cream border around the edge to create a dam, then spoon a generous layer of the chilled strawberry-raspberry filling into the center and smooth it out.
• Top with the next cake layer, press gently, and repeat the cream and filling layers until all layers are stacked.
• Coat the entire cake with the remaining mascarpone cream. Hold the cake over a plate or bowl and press the toasted sliced almonds onto the sides so they adhere.
• Use remaining piped cream to make decorative swirls on top and arrange fresh strawberries and raspberries if desired. Chill the assembled cake in the refrigerator until ready to serve.