01 - Line the bottom of a 9-inch springform pan with parchment paper.
02 - Pulse digestive biscuits in a food processor to fine crumbs. Combine with melted butter, mixing until evenly blended. Press mixture firmly into the prepared pan, creating an even layer, then chill in the refrigerator for 20 minutes.
03 - In a small saucepan, heat heavy cream until just before boiling. Pour hot cream over chopped dark chocolate in a heatproof bowl, letting it sit for 2 minutes. Add butter and stir until the mixture is silky and completely smooth.
04 - Pour prepared ganache over the chilled biscuit base, spreading evenly. Refrigerate for another 20 minutes to allow the ganache to set slightly.
05 - In a mixing bowl, beat mascarpone cheese, heavy cream, powdered sugar, and vanilla extract together until thick and smooth. Carefully spread the mixture over the set ganache layer. Chill for a minimum of 1 hour.
06 - In a bowl, gently toss mixed berries with sugar and lemon juice. Evenly spoon the berry mixture over the vanilla cream layer.
07 - Garnish the dessert with chocolate shavings and fresh mint leaves if desired. Carefully remove the dessert from the pan and slice for serving.