In a medium saucepan, boil the grape juice over moderately high heat until reduced by half; let cool. Transfer 1/4 cup of the juice to a small skillet. Sprinkle 1-1/2 teaspoons of the gelatin over the grape juice and let stand until softened, about 10 minutes. Warm the gelatin mixture over low heat, stirring once or twice until dissolved. Let cool, then stir the mixture back into the remaining cooled grape juice; reserve 3/4 cup of the grape gelatin and carefully pour the rest into 6 Champagne flutes. Refrigerate the flutes until the grape gelatin is chilled and set, at least 2 hours.
In a medium saucepan, combine the heavy cream, sugar and lemon zest and bring to a simmer. Let cool, then discard the lemon zest. Pour 1/4 cup of the cream into a small skillet and sprinkle the remaining 1-1/4 teaspoons of gelatin on top. Let stand until the gelatin is softened, about 10 minutes. Warm the gelatin mixture over low heat until dissolved, then stir it into the remaining cream along with the buttermilk; let cool completely.
Carefully pour the buttermilk panna cotta into the Champagne flutes and chill until set, about 2 hours. If necessary, rewarm the remaining grape gelatin over low heat until it's just pourable; pour a thin layer over the panna cotta in each flute. Refrigerate until set. Serve the parfaits with amaretti.:
Make Ahead: The parfaits can be refrigerated for up to 2 days.