Prepare the passion fruit base: Halve the passion fruits and scoop out the pulp and seeds into a fine-mesh sieve set over a bowl. Gently press the pulp through the sieve with a spoon to extract the juice, discarding the seeds (or reserve a few for garnish). You should aim for about 1/2 cup of pure passion fruit juice. If using frozen pulp, thaw it and strain as needed. Stir in the granulated sugar and lemon juice (if using) into the passion fruit juice until the sugar is dissolved. Set aside.

Whip the cream: In a separate, chilled mixing bowl, combine the cold heavy whipping cream, powdered sugar, and vanilla extract. Using an electric mixer or a whisk, beat the cream until stiff peaks form. Be careful not to over-whip, or it will turn buttery.

Combine the mixtures: Gently fold about one-third of the whipped cream into the passion fruit mixture to lighten it. Then, carefully fold the lightened passion fruit mixture into the remaining whipped cream until just combined. Avoid over-mixing, as this can deflate the mousse.

Chill the mousse: Spoon the mousse into individual serving glasses, ramekins, or a single larger serving dish. Cover with plastic wrap, ensuring the wrap doesn't touch the surface of the mousse if possible, and refrigerate for at least 2-3 hours, or until set.

Serve: Before serving, garnish with fresh mint sprigs and a sprinkle of reserved passion fruit seeds, if desired. Enjoy the tropical delight!