Place the milk in a medium saucepan; slowly whisk in the gelatin. Let stand until the gelatin softens, about 4 minutes.
Meanwhile, place the strawberries in a blender or food processor and puree until smooth. Press the puree through a fine-mesh sieve and discard the seeds.
To the saucepan with the gelatin mixture, add the strawberry puree, heavy cream, sweetener, extract, and salt. Whisk over medium heat just until the gelatin dissolves, about 2 minutes.Remove the pan from the heat. Divide the panna cotta among cute dessert dishes or ramekins. Cover and chill until set, at least 5 hours or overnight.
Serve garnished with sliced strawberries, if desired. Store extras covered in the refrigerator for up to 4 days. Do not freeze.