Prepare the ramekins:

Spray 8 (4-ounce) ramekins with nonstick spray to ensure that the panna cotta unmolds easily later.

Prepare the gelatin:

In a small bowl, pour 3 tablespoons of milk and sprinkle the unflavored gelatin over the top. Let it sit for about 10 minutes to allow the gelatin to bloom.

Heat the milk and dissolve gelatin:

In a small saucepan, heat the remaining 1 1/4 cups of milk and 5 tablespoons of sugar over medium heat, stirring occasionally. Once the milk is warm, add the gelatin mixture and stir until both the sugar and gelatin are completely dissolved. Be careful not to boil the mixture.

Whisk in yogurt and vanilla:

Remove the milk mixture from the heat and let it cool slightly. Then, whisk in the Greek yogurt and vanilla extract until smooth and well combined.

Divide the panna cotta mixture:

Divide the yogurt mixture equally among the prepared ramekins, using about 1/3 cup per ramekin. Cover each ramekin with plastic wrap and chill in the refrigerator for at least 6 hours, or preferably overnight, until set.

Macerate the berries:

About 30 minutes before serving, combine the mixed berries with the remaining 1 tablespoon of sugar in a medium bowl. Stir occasionally with a rubber spatula to help release the juice from the berries. Let them sit for at least 10 minutes to macerate and develop a syrupy consistency.

Unmold and serve:

To serve, gently unmold each panna cotta by running a knife around the edges, inverting each ramekin onto a small plate. Top each panna cotta with about 1/2 cup of the macerated berries and garnish with fresh mint leaves, if desired.