In a medium saucepan, combine fresh blackberries, a splash of Grand Marnier (if using), fresh lemon juice, and granulated sugar. Place over medium heat and bring to a rolling boil. Reduce heat and simmer gently for 8-10 minutes until thickened. Remove from heat and cool completely.

While syrup cools, in a small bowl, combine unflavored gelatin with cold water, stirring to moisten. Microwave for 15 seconds until dissolved. In a larger bowl, whip heavy cream until soft peaks form, then gradually add dissolved gelatin, powdered sugar, and vanilla. Whisk until smooth and fluffy.

Gently fold plain Greek yogurt into the whipped cream mixture until fully combined.

To assemble, layer cooled blackberry syrup and creamy mixture in serving glasses. Repeat layers until filled, finishing with cream on top.

Cover with plastic wrap and refrigerate for at least an hour. Before serving, garnish with fresh blackberries and mint leaves.