Brush rose petals with egg white, dust with caster sugar, and dry overnight.
Process shortbread ingredients in a chilled bowl until just combined and press into a round.
Wrap dough in film and chill for at least one hour.
Preheat the oven to 150°C.
Roll dough to 1cm thickness, cut into rectangles, and chill on a baking sheet for 30 minutes.
Bake for 25-30 minutes until set without color and cool on a wire rack.
Heat double cream with vanilla pods to a boil, then stir in condensed milk.
Dissolve squeezed gelatine into the hot cream and cool mixture to body temperature.
Stir in buttermilk, strain through a sieve, and add scraped vanilla seeds.
Pour into 10cm moulds and refrigerate for at least four hours.
Simmer sugar, water, grenadine, and rosewater, then cook rhubarb in the syrup for 3 minutes.
Unmould creams onto plates and garnish with rhubarb, sauce, and crystallized petals.