Special equipment: 6 (6-ounce) stainless-steel or ceramic molds
Make panna cotta: Blend strawberries, buttermilk, and sugar in a blender until very smooth. Pour through a very fine sieve into a medium bowl, pressing hard on solids and scraping the bottom of the sieve. Discard solids.
Sprinkle gelatin over milk in a small bowl and let stand 1 minute to soften.
Bring cream to a boil in a small saucepan. Remove from heat and add gelatin mixture, stirring until dissolved.
Whisk cream mixture into strawberry puree and pour into molds. Chill molds, covered, until firm, at least 8 hours.
To unmold: Dip molds in a small bowl of hot water 2 or 3 seconds, then invert onto dessert plates and remove molds. Let stand at room temperature 20 minutes to soften slightly.
Make compote while panna cottas stand: Halve strawberries lengthwise if small, or quarter if larger. Whisk together orange juice and superfine sugar in a bowl until sugar is dissolved and add strawberries, tossing to coat. Set the compote aside to macerate for 10 minutes.
Serve panna cotta with compote.
Panna cotta can be chilled in molds, covered, up to 2 days.