The beauty of this dessert is in its simplicity (and the fact that you don’t have to turn on the oven). Lightly flavoured cooked cream is cool and refreshing alongside any summer fruit, so as the season progresses you can serve this with whichever fruit is at its peak.
Warm the half-&-half and whipping cream in a small pot over medium heat with the sugar, lemon zest and vanilla until the sugar has fully dissolved, about 3 minutes. While the cream is heating, soften the gelatin in 2 Tbsp cold water.
Add the softened gelatin to the cream and whisk until the gelatin has dissolved (there will be no noticeable change in texture). Remove the pot from the heat and whisk in the buttermilk. Ladle or pour this into 6 lightly oiled ramekins or other dishes and then chill the panna cottas for at least 4 hours.
To serve, run a knife around each panna cotta to loosen it and then turn the ramekin onto a plate to tip the dessert out. Serve the panna cotta with fresh fruit.