Lightly grease eight 4-oz. ramekins or cups with nonstick cooking spray.
Puree strawberries and sugar in a blender until very smooth. Press through a sieve into a bowl; discard seeds. Stir in vanilla; Remove 1 cup for the panna cotta; reserve remaining puree.
Sprinkle gelatin over 1/4 cup water in a small custard cup; let stand 5 minutes to soften. Place cup in a small skillet of simmering water; stir until gelatin is dissolved. Simmer milk in saucepan. Remove from heat; Transfer to a 1-qt. glass measure; stir in reserved 1 cup strawberry puree and the cream until well blended. Pour into ramekins. Chill, covered, until firm, about 4 hours or up to 2 days.
To unmold, run a small knife around each; invert onto plates, gently shaking to remove. Spoon some of the reserved strawberry sauce onto panna cotta. Garnish with sliced strawberries.