In a medium saucepan, combine 1 cup of sugar and 1 cup of water. Heat over medium heat, stirring until the sugar is fully dissolved. Once dissolved, remove from heat and allow to cool slightly. Stir in 1/2 cup of Grand Marnier. Set aside.
Brew 1 cup of strong coffee. Allow it to cool to room temperature. Optionally, add a splash of Grand Marnier to the coffee.
Rinse and slice the mixed berries, setting aside a few for garnish later.
In a mixing bowl, whip 1 cup of heavy cream until stiff peaks form. Set aside.
In another bowl, combine 1 cup of mascarpone cheese and 1 teaspoon of vanilla extract. Gently fold in the whipped cream until fully combined.
Dip each ladyfinger into the coffee mixture briefly, then layer them in the bottom of the baking dish. Spread half of the mascarpone mixture over the ladyfingers, followed by half of the mixed berries. Repeat the layers, finishing with the remaining mascarpone mixture on top.
Cover the tiramisu with plastic wrap and refrigerate for at least 4 hours, or overnight for the best flavor.
Before serving, garnish with reserved berries and a dusting of cocoa powder if desired. Slice and serve chilled.
