Make panna cotta:
Pour 1 cup of buttermilk into the top of a double boiler (not over heat). Sprinkle gelatin over the buttermilk, let stand to soften, about 5 minutes.
Meanwhile, in a small saucepan, bring cream and scant 1/2 cup sugar to a boil. Add cream mixture to the gelatin mixture; place over simmering water; whisk until the gelatin dissolves, about 5 minutes. Stir in the remaining cup of buttermilk. Pass the mixture through a cheesecloth-lined strainer. Divide among six 4-ounce ramekins or small bowls on a baking sheet. Cover; refrigerate until set, about 4 hours.
Place 1/4 cup of the lemon juice in a mixing bowl. Sprinkle the gelatin over the lemon juice and let stand to soften, about 5 minutes.
In a small pan bring the sugar and 1 cup of water to a boil over high heat. Pour the syrup over the gelatin mixture, whisk to dissolve. Add the remaining 1/4 cup of lemon juice. Allow the mixture to return to room temperature.
Once the buttermilk panna cotta has set, pour a 1/4-inch-thin layer of lemon jelly on top of each ramekin. Refrigerate until set, about 30 minutes. The panna cotta ramekins can be prepared up to 24 hours in advance, covered and refrigerated. Serve chilled and garnish with lemon sorbet and crispy cookies.