01 - Strain the fresh watermelon juice through a fine mesh sieve to remove all pulp and seeds, ensuring a smooth liquid base.
02 - In a medium saucepan, whisk together the granulated sugar and cornstarch until thoroughly blended and no lumps remain.
03 - Gradually pour the strained watermelon juice into the saucepan while whisking continuously. Stir until the mixture is completely smooth and free of any starch lumps.
04 - Mix in the fresh lemon zest and vanilla extract or powder, ensuring even distribution throughout the liquid.
05 - Place the saucepan over medium heat, stirring constantly with a whisk to prevent scorching. Cook for 8-10 minutes until the mixture thickens to a soft pudding consistency that coats the back of a spoon.
06 - Remove from heat immediately. If using, fold in the dark chocolate chips now, allowing them to soften slightly for the traditional seeded appearance.
07 - Pour the warm pudding into individual serving glasses or molds. Allow to cool to room temperature, then refrigerate for at least 3 hours until completely set and firm.
08 - Just before serving, generously sprinkle the tops with finely chopped pistachios. Add dried rose petals and extra chocolate chips as desired for visual appeal.