Prepare the Berries: Wash and gently dry the strawberries, blueberries, raspberries, and blackberries. Slice the strawberries and reserve some of the best-looking berries for the topping.
Make the Berry Mixture: Combine the berries with powdered sugar, lemon juice, and lemon zest. Gently toss and let the mixture sit for several minutes so the berries release some of their natural juices.
Prepare the Chantilly Cream: Beat the mascarpone until smooth. Add the powdered sugar and vanilla extract and mix until combined.
Whip the Cream: In a separate bowl, whip the cold heavy cream until soft peaks form. Gently fold the whipped cream into the mascarpone mixture until smooth, fluffy, and creamy.
Dip the Ladyfingers: Quickly dip each ladyfinger into the berry juice or berry puree. Avoid soaking them for too long because they can become too soft.
Build the First Layer: Arrange the soaked ladyfingers in an even layer across the bottom of the baking dish. Break some pieces if needed to fill the gaps.
Add the Chantilly Cream: Spread half of the Chantilly mascarpone cream evenly over the ladyfingers.
Add the Berries: Scatter a layer of the prepared fresh berries evenly over the cream.
Repeat the Layers: Add another layer of berry-soaked ladyfingers, followed by the remaining Chantilly cream. Smooth the top with a spatula.
Chill: Cover the tiramisu and refrigerate for at least 4 hours, preferably overnight, allowing the flavors and textures to develop.
Garnish and Serve: Just before serving, decorate with the reserved fresh berries and optional mint leaves. Serve chilled.