In a small bowl, sprinkle the unflavored gelatin over 2 tablespoons of cold water. Let it sit for about 5 minutes to bloom.
In a medium saucepan, combine the whole milk, heavy cream, 1/2 cup of sugar, and a pinch of salt. Heat over medium heat, stirring occasionally, until the mixture is hot but not boiling.
Once the milk and cream mixture is hot, remove it from the heat. Add the bloomed gelatin and stir until fully dissolved. Then, stir in the vanilla extract.
Allow the mixture to cool slightly before whisking in the sour cream until smooth and well combined.
Carefully pour the Panna Cotta mixture into individual serving dishes or molds. Cover with plastic wrap and refrigerate for at least 4 hours, or until fully set.
While the Panna Cotta sets, prepare the berry topping. In a medium bowl, combine the 2 cups of berries, 1/4 cup of sugar, and lemon juice. Gently toss to coat the berries and let them sit for about 30 minutes to create a syrup.
Once the Panna Cotta is set, remove it from the refrigerator. If using molds, dip the bottom of each mold in hot water for a few seconds to loosen the Panna Cotta before inverting onto serving plates. Top with the fresh berry mixture and serve immediately.