Combine 3/4 cup of the milk and cornstarch in a bowl and stir until smooth.
Pour remaining milk into a pot and stir in sugar and cardamom extract. Heat over low heat, stirring until sugar is fully dissolved. Whisk in cornstarch mixture and simmer until thickened, 5 to 10 minutes.
Pour milk mixture into 5 ramekins, cover, and cool to room temperature. Chill in the refrigerator for at least 3 hours.
Combine water, sugar, and vanilla seeds in a pot and bring to a boil over low heat. Stir until sugar is dissolved. Allow syrup to cool. Sprinkle each serving with pistachios and rose petals. Drizzle with 1 tablespoon syrup.