In a small saucepan, combine 1 cup of sugar and 1 cup of water. Bring to a gentle boil, stirring occasionally until the sugar dissolves. Remove from heat and let cool. Once cool, stir in 1 cup of Grand Marnier.
Wash and pat dry the mixed berries. If using strawberries, slice them into halves or quarters, depending on their size. Set aside.
In a mixing bowl, whip 1 cup of heavy cream until soft peaks form. In another bowl, mix 1 cup of mascarpone cheese and 1 teaspoon of vanilla extract until smooth. Gently fold the whipped cream into the mascarpone mixture until well combined.
Quickly dip each ladyfinger into the Grand Marnier syrup, ensuring they are soaked but not soggy. Lay half of the soaked ladyfingers in a single layer in your 9x13-inch baking dish. Spread half of the mascarpone cream over the ladyfingers, followed by half of the mixed berries. Repeat the layers, finishing with a layer of mascarpone cream.
Cover the baking dish with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.
Before serving, garnish your Berry Tiramisu with fresh mint leaves and a sprinkle of additional berries for an eye-catching presentation. Slice into squares and serve chilled.
