In a medium bowl, combine sliced strawberries and blueberries with 2 tablespoons granulated sugar. Toss gently and let sit for 30 minutes to macerate and release their juices.

In a large chilled mixing bowl, beat the heavy cream with an electric mixer on medium-high speed until soft peaks begin to form, about 2-3 minutes.

Add the mascarpone cheese, powdered sugar, remaining 1 tablespoon sugar, and vanilla extract to the whipped cream. Continue beating on medium speed until stiff peaks form and the mixture is smooth and spreadable, about 1 minute more.

In a trifle bowl or large glass serving dish, arrange one-third of the cake cubes in an even layer on the bottom.

Spoon one-third of the mascarpone cream over the cake layer, spreading it gently to the edges. Top with one-third of the macerated berries and their juices.

Repeat the layering two more times: cake cubes, cream, then berries. For the final layer, arrange the prettiest whole berries in a decorative pattern on top.

Cover the trifle with plastic wrap and refrigerate for at least 2 hours or up to 24 hours before serving. The cake will absorb the berry juices and soften beautifully as it chills.

Just before serving, add any reserved fresh berries as a final garnish. Scoop through all layers when serving to ensure everyone gets cake, cream, and fruit in each portion.