Make the passion fruit curd first: In a small saucepan, whisk together passion fruit pulp, 1/4 cup sugar, and egg yolks over medium-low heat. Cook while stirring constantly until the mixture thickens and coats the back of a spoon, about 5-7 minutes. Remove from heat, stir in the butter until melted and smooth, then transfer to a bowl. Cover with plastic wrap pressed directly on the surface and refrigerate until completely cold, at least 2 hours.

In a medium saucepan, whisk together the sweet rice flour, 1/3 cup sugar, and water until completely smooth with no lumps. Place over medium heat and cook, stirring constantly with a silicone spatula, until the mixture thickens into a glossy, stretchy dough that pulls away from the sides of the pan, about 5-6 minutes.

Generously dust a clean work surface with potato starch. Turn the hot mochi dough out onto the starch and dust the top heavily. Let cool for 5 minutes until just warm enough to handle safely.

Divide the dough into 8 equal portions using a bench scraper or knife dusted with starch. Keep unused portions covered with a damp towel to prevent drying.

Wet your hands with cold water, take one portion and flatten it into a 3-inch circle in your palm. Place 1 tablespoon of chilled passion fruit curd in the center.

Gather the edges of the dough up and over the filling, pinching firmly to seal. Roll gently between your palms to smooth, then place seam-side down in a potato starch-dusted container.

Repeat with remaining portions, rewetting your hands as needed. Dust the finished mochi lightly with additional potato starch, shaking off excess. Let them rest at room temperature for 30 minutes before serving to allow the texture to set fully.