Make panna cotta: In a bowl, sprinkle gelatin over 1 cup buttermilk; let rest 5 minutes.
Whisk sugar and half-andhalf in a saucepan over medium heat to dissolve sugar. When mixture begins to bubble at edges, pour over buttermilk mixture and whisk to combine and dissolve gelatin. Stir in remaining buttermilk. Strain mixture through a fine-meshed sieve and divide evenly among six 6 oz. ramekins. Refrigerate panna cotta until set, at least 6 hours or overnight.
Make sauce: Blend raspberries with juice and sugar in food processor until smooth. Strain; discard solids.
To loosen panna cotta, run a thin-bladed knife around edges and invert onto plates. Serve with raspberry sauce.