Place the sliced strawberries and blueberries in a bowl, then sprinkle 1/4 cup of the granulated sugar over them. Gently mix and let the fruits sit for about 15 minutes to release their juices.

In a separate bowl, combine the cooled coffee with the remaining 3/4 cup of sugar and the vanilla extract, stirring until the sugar has completely dissolved.

In a large mixing bowl, whip the heavy cream until it forms soft peaks. Carefully fold in the mascarpone cheese until the mixture is smooth and creamy.

Quickly immerse each ladyfinger in the coffee mixture for just 1-2 seconds, then layer them in the bottom of a 9x13 inch dish.

Spread half of the mascarpone cream over the ladyfingers, followed by half of the macerated strawberries and blueberries. Repeat this layering process with the remaining ladyfingers, mascarpone cream, and berries.

Cover the dish with plastic wrap and refrigerate for at least 4 hours, or ideally overnight.

When ready to serve, lightly dust the top with cocoa powder.