In a small bowl, mix 2 tablespoons of cold water with 10 grams of unflavored gelatin powder. Let it sit for 5 minutes to bloom.
In a saucepan, heat 500 ml of full-fat milk and 250 ml of fresh cream over medium heat. Add 100 grams of sugar and stir until it dissolves completely. Do not let the mixture boil.
Remove the saucepan from heat and add the bloomed gelatin. Stir until the gelatin dissolves completely.
Pour the mixture into individual serving glasses or ramekins, leaving some space for the blackberry layer. Refrigerate for at least 4 hours or until the panna cotta is set.
In another saucepan, add 250 grams of fresh or frozen blackberries, 50 grams of sugar, and 2 tablespoons of water. Cook over medium heat until the blackberries are soft and the sugar has dissolved.
Blend the blackberry mixture until smooth and strain it through a fine-mesh sieve to remove the seeds.
Allow the blackberry puree to cool to room temperature. Once the panna cotta is set, pour the blackberry puree over the top of each serving glass or ramekin.
Refrigerate for another 1-2 hours or until the blackberry layer is set. Garnish with fresh blackberries and mint leaves before serving.