1. Cut tops off of passion fruits and use a spoon to scoop out the pulp and seeds (you should have about 1 cup pulp, including seeds).
2. In a large bowl, combine heavy cream, sugar, and Grand Marnier and beat with a mixer on high speed until soft peaks form.
3. Gently fold all but 2 tbsp. of the passion-fruit pulp and seeds into the cream mixture.
4. Divide passion-fruit cream mixture among 8 parfait glasses, drizzling each with a bit of the reserved pulp and seeds.